Both blue and red must KEEP CLEAR of yellow. End of story, and if it was just a scenario between blue and yellow, I have little doubt that is exactly what would happen (blue would likely tack). But blue cannot tack to keep clear because red is to windward of her – so blue needs to call for “room to tack” from red or she needs to duck (pass astern) of yellow. If she ducks she won’t make the mark – not what she would want to do.

Red, being the windward boat to blue, must KEEP CLEAR of both yellow and blue. So there are rule 10, 11, 13, and 17 considerations here. Your initial analysis is absolutely right – but go a little further with it and see what you come up with. Especially considering what red/blue must/could do, and back up to position three and four and think specifically how blue might have defended against the slam dunk.

To answer your follow up question, if red and blue do not KEEP CLEAR – or as you put it give way – then yellow must avoid contact with these two boats (tack, duck or slow down). After that she should immediately hail protest AND immediately fly her protest flag to protest the infringement of Rule 10. That said, if yellow doesn’t protest (which she should) red and blue should both still take a penalty if in fact they broke a rule (which in this scenario seems obvious that they did). After that red or blue would have to protest one another if they believed they were COMPELLED to break the rule by the other boat’s actions.