Niko there is an old thread that we discussed this (used to be a Case or a Q&A too, but was withdrawn) ... . but your OP is different with the boat calling for room to tack not being given room to complete her tack. 

I'll try to find it but if anyone else can put your fingers on it, please post a link. 

A..yes i believe it is see 20.2(e)  position 0 has no effect.
B. Yes I believe she can. there is a definition issuer here though.  Yellow does tack onto starboard, but this has noting to do with her completing her tack this is just an issue as to when RRS 13 ceases to apply.  Another interesting question is does 15 apply Green gets ROW because yellow asked her to tack so perhaps no 15?.
C. as green and yellow passed head to wind in the zone, and blue had to go above close-hauled both should be dsq. 

Note: I did not see blue as "well overstood"

This is an interesting variation on the old ‘meat in the sandwich’ case which was withdrawn and never to my knowledge, resolved.

The hail for room to tack turns of the mark room for Green 20.2(e). Once Green tacks, Yellow is obligated to tack as soon as possible, which she does aggressively. As both are tacking R 13 requires Green on Yellows port side, to stay clear – which she fails to do. So Green breaks R 13 with no exoneration. She failed to allow Yellow to complete her tack and breaks R 20.2(c).

Yellow breaks R 18.3 relative to Blue, and in my opinion should be exonerated as she was not given room when Green failed to stay clear.

John

I do not see yellow as failing to keep clear.
I see no reason for green to be exonerated.
Green did not have mark room 18.2(d) so cannot be exonerated for being in her mark room.
She was not compelled to break a rule as she could have passed to leeward of the mark.

vert était au mauvais endroit au mauvais moment, il devais passer sous la marque qu'il ne pouvait parer sans aller bout au vent

Facts found
Blue, close-hauled on starboard, outside the zone, was fetching a mark to be left to port, on course to pass about I hull length from the mark.
Yellow, close-hauled on port was on a converging course with Blue.
Green, close-hauled on port entered the zone, overlapped inside Yellow.
Yellow hailed for room to tack.
Green luffed, and immediately afterwards Yellow luffed.
Green passed head to wind and reached close-hauled, on a course that would not allow her to pass the mark without luffing.
Yellow passed head to wind but had not reached a close-hauled course. Yellow was now overlapped, about a metre (or less ?) to windward of Green.
Blue, now inside the zone, was overlapped a hull width to windward of Yellow.
Green luffed, and was now on a course to pass close to windward of the mark. Yellow remains above close-hauled and is now a metre to windward of Green. . Blue, now on a converging with Yellow, luffs above close-hauled and passes one metre to windward of Yellow.

Conclusions
When Green passes head to wind from port to starboard Blue is fetching the mark. RRS 18.2 does not apply between them, 18.3 does.
When Yellow passes head to wind from port to starboard, both Blue and Green are fetching the mark, RRS 18.2 does not apply and 18.3 does.
When Green, over lapped to leeward luffs to pass the mark she is a right of way boat changing course. RRS 16.1 applies and Green must give Yellow room to meet her obligation under RRS 18.3 not to cause Blue, on starboard since entering the zone, to sail above close-hauled.
Yellow causes Blue, on starboard since entering the zone, to sail above close-hauled. Yellow breaks RRS 18.3(a).  
Green has not given Yellow room to meet her obligation under RRS 18.3 not to cause Blue, on starboard since entering the zone, to sail above close-hauled. Green breaks RRS 16.1. Yellow is exonerated under RRS 43.1.
Decision 
Green DSQ

Discussion
In my opinion the key fact is that between position 3 and 4 Green luffs and is subject to RRS 16.1. As a result Yellow cannot bear away to let Blue pass to windward without sailing above close hauled.

Reminds me of withdrawn case 133

Why does 16.1 apply?
I know this a strange area or the rules and the fetching boat and the two others are not subject to 18.2.
The inner boat surely has mark room ans 16.1 indemnity under 18. 2(c).

Mike ...    neither former port tack boats (G or Y) are entitled to MR under the new 18.3.  We've had a couple threads about this.

PS: the link to the old thread comment. 

I agree on blue, but why not relative to each other as neither were fetching the mark on starboard? 

Mike I included a link to the play-by-play description above (from a previous thread) .. but here is it below cut and pasted for everyone's convenience.
Created: 25-Jan-20 10:19

\

I await the rule change to get rid of the nonsence ws have created. 

Reinhard, I wonder if the new 18 and 18.3 "fix" the Case 133 issue? (Or make it clearer).

Can someone paste the image here from the 2013-2016 Case Book? (Links to old images on RRoS got broken in a past update)

The text was as follows:

Case 133 

Definitions, 18.3, Mark Room: Tacking in the Zone
Rule 21, Exoneration
Rule 64.1(a), Decisions: Penalties and Exoneration

Analysis of the application of rule 18.3 to a situation at a windward mark to be left to port in which two port-tack boats tack in quick succession to leeward of a starboard-tack boat that is fetching the mark. Both of the boats that tacked broke rule 18.3.

Assumed Facts
Boat M does not hail for room to tack. Shortly after position 2, Boat I decides she can tack and fetch the mark. Just after I luffs to tack, M begins to tack. I passes head to wind shortly before M does. As the boats tack, Boat S continues to sail a close-hauled course and there is space for one boat (but not two) to pass between S and the mark. Between positions 4 and 5, I luffs to round the mark. In response to I’s luff, both M and S luff above close-hauled. There is no contact. No boat takes a Two-Turns Penalty. Both S and M protest I.

Question
How does rule 18.3 apply to this incident and what should the decision be?

Answer
The two protests were the result of a single incident, so they should be heard together.
S broke no rule. She was required by rule 11 to keep clear of M from the moment M completed her tack, and she did so.

After I and M passed head to wind, each of them was on the same tack as S who was fetching the mark, so rule 18.3(a) applied to both I and M with respect to S. When both I and M luffed between positions 4 and 5, S sailed above close-hauled to avoid contact, so both I and M broke rule 18.3(a). However, M is exonerated under rule 64.1(a) for breaking rule 18.3(a) because she was compelled to luff by I’s luff.

After I changed tack she was fetching the mark. When M changed tack she became overlapped outside I, so rule 18.3(b) applied to M. It required M to give I mark-room, including ‘space to comply with her obligations under the rules of Part 2’ (see the definitions Room and Mark-Room). When I luffed to sail to the mark, she broke rule 18.3(a). Because M did not give I space to comply with rule 18.3(a) as I sailed to the mark, M broke rule 18.3(b).

Note that I is not exonerated for her breach of rule 18.3(a). She is not exonerated under rule 64.1(a) because she was not compelled to luff between positions 4 and 5. Instead she could have borne off after position 4 and left the mark to starboard. She is not exonerated under rule 21 because rule 18.3(a) is not one of the rules listed in rule 21(a).

To sum up, I broke rule 18.3(a) by causing S to sail above close-hauled and she is not exonerated for that breach. M broke rule 18.3(b) by failing to give I mark-room and she is not exonerated for that breach. Therefore, both I and M are disqualified.

Case 133 was published in the 2015 supplement.
Download here   https://www.sailing.org/tools/documents/20132016ISAFCaseBookSupplement2015-[16130].pdf

and here is the diagram

John

Green tacked first so yellow having to tack was not fetching or on starboard so surely 18.2 applies between them and Green gets mark room when she becomes inside overlapped and an indemnity from 16.1.
The previous thread did not seem to incorporate this if it was under these rules.

So there is a change now with the new rules in Case 133. 

M (Yellow in the OP) no longer breaks a MR rule. 

Maybe they'll rework it and republish it. 

Mike, if Green tacked first, then when Yellow tacks Green is on starboard fetching the mark.  Therefore 18.3 applies between them and 18.2 does not. 

If Green is in a position to shoot the mark and round without tacking, she is fetching it. 

Mike said:  I await the rule change to get rid of the nonsence ws have created. 
I would argue that the new organisation of the rule resolves numerous issues. It clearly states when 18.2 switches off and separates this from the obligations of the tacking boat if a boat was on starboard since entering the zone.
In this case, when Green passed head to wind from port to starboard, Blue was on starboard and fetching the mark. RRS 18.2 does not apply between them.
Yellow luffs and passes head to wind after Green. Green is now in a position to pass to windward of the mark and leave it on the required side without changing tack. Green is on starboard and fetching the mark.   RRS 18.2 does not apply between Green and Yellow. 
When Yellow passed head to wind from port to starboard, Blue was on starboard and fetching the mark. RRS 18.2 does not apply between them. 
As Green is not entitled to mark room when she luffs to pass the mark she must give room to Yellow to keep clear and comply with her obligations under the rules of part 2 (see definition room), in this case 18.3(a).   Yellow cannot bear away to let Blue pass to windward because Green has luffed. Blue luffs to avoid contact with Yellow and sails above close-hauled. Yellow breaks RRS 18.3(a) as a result of Green not giving room as required by RRS 16.1. Yellow is exonerated under RRS 43.1(a).

If 18.3 applies does the fetching boat overlapped inside not get mark room?

Mike re:"If 18.3 applies does the fetching boat overlapped inside not get mark room?"

No, not between Yellow and Green. 18.2 is off between them and neither boat enters the zone on starboard. 

Under the new 18.3, the only boat that can possibly get MR (in the OP scenario) is if the boat on starboard fetching the mark also entered the zone on starboard and then becomes overlapped inside the boat that tacked .. under 18.3(b). 

So Blue could get MR from Yellow and/or Green if Blue became overlapped inside of them. 

But since neither Yellow or Green entered the zone on starboard, 18.3(b) cannot apply between them. 

This is one of the issues resolved by the new wording. The new rule makes clear that rule 18.2 is switched off.
Under the old rule, once both Yellow and Green passed head to wind from port to starboard in the zone, Yellow would have had to give mark-room to Green under 18.2(a) but not cause Blue to sail above close-hauled under 18.3.

If R18 is turned of , how about R11 leeward/windward boat between Green and Yellow?. Yellow must keep clear of a leward boat on the same tack with ROW and forces blue to luff, breaking R18.3
If Yellow wasn't present or passed astern of Blue, there would not have been any problems. So yellows presence and action causes all the fuss. By tacking later than Green she immediately becomes windward boat (of Green) and must keep clear even though she is still bound by R13, which Green has already cleared.
Yellows only gets room to tack (HTW) not necessary room to reach close-hauled course
DSQ : Y
The new R18 rewording still seems to be a mess.

In writing the rule, maybe we have to define the goal first (DSQ Y, DSQ Y&G or whatever) and then define the rule so this will be the outcome.

I agree with many here and think this complex but clear. Because Green must give Yellow room to keep clear (would have to do so by 15, even if 16.1 were not turned on) AND does not have mark-room, she is quite limited. 18.3 regarding Blue is Green's #1 priority, including giving room for Yellow to also not violate 18.3.

We know the rules try to discourage port-tack approaches, and this is one more way that they do this, empowering the slightly less port-tacky approach.
Indeed for the most part, I think instinctively, we know that Blue is alpha, and Yellow, by virtue of being in the middle is next in line in terms of how the rules support her and allow her an out.

Here is what I come up with:

        Yellow –13, 18.3(a)
        Green – 16.1, 18.3(a)

        Blue – None

        Yellow – exonerated for breaking 13 & 18.3(a).

        Green – no exoneration.

        Blue – no exoneration needed.

Green is DSQ.

Question:
Does the requirement in 20.2(c) to give the hailing boat “room to tack and avoid her” only apply when the hailed boat responds ‘you tack’ or does it also apply when the hailed boat tacks immediately?

This PDF is my detailed step-by-step analysis - not for the faint of heart!

I think the discussion is moving away from the question in the OP.
My take is simple -
Blue is an obstruction R 19.
Yellow hails correctly for room to tack under R 20.
Green responds by tacking.
Yellow begins to tack.
Green's obligation under 20.2(c) is to allow Yellow to tack - which includes Yellow coming to a close hauled course.
At P3 and at P4 Green prevents Yellow from completing her tack as required by R20 so she is not giving room to tack..

We can argue about whether Green also broke R 13 but it is moot! Green is DSQ under R 20.2(c) and Yellow may be exonerated from her breach of 18.3 relative to Blue.
John