So at some point you made a commitment to both of them that if there were issues you would leave. A is expecting you to live up to the commitment you made. So why do you not feel that you can live up to the commitment you made?
If it's because B is not happy with that commitment, that's not your problem, but his.
When you say "we" are trying to create an equal "V", it's sounding more like two of you want to but the third absolutely does not. That isn't going to work, no matter how hard you try. In fact, the more energy you put into it, the worse things are going to get.
I agree with other posters - this isn't going to work.
Please check out The Birdcage - an open, friendly Polyamory forum for all parts of New York State
"Listen, or your tongue will make you deaf." - Native American Proverb