>>3I think you do make some really good points, but I don't think OP and the boy A are even dating, they just have mutual feelings for each other? Regardless, you bring a really great perspective up and I hope that OP does consider it.
And I've definitely been through my friends dating someone I liked and I currently have sorta a thing but not really with a friend I know at least two of my friends also have liked, and yeah, it's definitely not a fun thing to deal with. But if you're really friends, you can deal with it maturely and minimizing hurt feelings.
The big thing? Communication. If you communicate these things, it makes it much easier to deal with. It's the big reason why my friends are still friends and although there's gonna be a bit of jealousy no matter what, we still have respect for each other and are happy for each other.
Have a serious talk with both of your friends, OP. As the reply before me, you can do this separatly or all three of you together, whichever is better for you. But let them know how you feel about the situation and get their feelings and perspective on it as well. Make decisions from there.
At the same time though, if I'm correct that you two aren't even dating, friend B's behavior just seems really toxic and if I had a friend acting like that just because someone I'm not even dating happened to like me, I'd question their friendship. Friend B's behavior would make more sense if y'all are actually dating, but it still does not excuse it.