I don't think you have addressed what for me is a key part of this yet (and if you have, then please point me at it, because I am missing it).
When the three of you got together you made a commitment that, should anything go wrong, you would leave. Since that point, there has been no discussion among all three of you (since the promise was made with you all) about changing that. What it sounds like is that you and B have decided together that it's not going to be that way, but A never agreed to that.
Have I got that right?
"Listen, or your tongue will make you deaf." - Native American Proverb