Question 1
Three boats A, X and B, all on port tack, are approaching a gybe mark directly astern of each other. B was clear astern of X when X came within two lengths of A. A stops just inside the two-length zone. B bears away onto a course to pass to leeward of X. X bears away to pass to leeward of A. B and X become overlapped and B protests. What should the call be?

Answer 1
A is an obstruction because both X and B are required to keep clear of her under rule 18.2(c). B is clear astern of X when X enters A's two-length zone, so B must both keep clear and give X room (rule 18.2(c)). Under rule 17.2, X must not sail below her proper course, but sailing to leeward of A is a proper course.
No penalty.

Question 2
If X and B are overlapped when X reaches the two-length zone of the mark, how does this limit the boats’ actions?

Answer 2
A has right of way over both X and B under rule 18.2(c); each of them must keep clear of A until she has passed the mark, and is not entitled to room from A. B must give room to X to pass A.
X is subject to rule 18.2(a) (and 18.2(b) if the overlap is broken while the boats are rounding) with respect to B at the mark and must give room to B until both have passed the mark.
The presence of A may mean both boats have to pass the wrong side of the mark. X’s obligation to give B room continues until either both boats have passed the mark, or they are no longer 'about to round'.