OK, Rocket, interestingly, as you say, the road markings don't really change the picture in this case, what does change things is the introduction of the third car, C. Which way they are signalling to exit is totally irrelevant in all cases, for those of you still confused: it's not where you plan to exit, it's where you enter that matters. In Rocket's new picture, suddenly priorities are clear: B is to the right of C, and C is to the right of A, so A enters last of the three.
Hopspot the third entrance is irrelevant if no car is in it, just as the lack of a fourth entrance is irrelevant.
I do agree that if there is a whole queue behind B or C people are about to get both confused and annoyed, because no one will be clear on which car arrived before which other. Which is why, as your friendly driving instructor, I would remind you: this is not a race. Please do not insist on taking priority where an accident might occur. Just, like, live with one another. (Did I mention I've been driving, professionally, in London for 40 years?)