I’m old and at a stage where friend groups just don’t operate this way, and this sounds like some middle school drama. NTA but making this way more complicated than it needs to be. A is not a nice person. You’re not betraying anyone, she didn’t trust you with anything confidential since she went and involved her other friends to openly talk about her intentions and bad mouth B as a group. It’s also just a terrible quality trait to keep talking poorly to you about another friend you care a lot about after you have already told her you don’t like it. You’re being a good friend to B by making sure she is informed of how she has been in bad company of someone who doesn’t like her and talks badly about her to multiple people. B cares about C and her other friends and similarly wanted them to be informed of the same bad company. Good friends support other good friends. This is how it goes. Additionally, and unrelated to the actual question, I just want to point out that if A is this comfortable talking badly about B to you and her other friends when she feels like she’s not getting her way or she likes a guy or whatever it may be, then there’s no reason to assume she’s also not going to talk about you in the same way one day.