Friends have been acting irritatingly recently (I know that's not a question lol)?

I know no one will read the entire thing or probably answer since I'm just technically just ranting, haha.

I have this one friend (I'll call her Friend A) who constantly clings to another one of my friends (Friend B). Friend A used to be friends with all of my other friends, but A dislikes them now (and seems to have taken a liking for talking s*** about them) and only talks to B and me. A seems to have been "diagnosed" (self-diagnosed) with anxiety and depression and though it's true that A does have a somewhat saddening background, A needs to get over it because it's common in the world. Though I've supported A for a longer time than B, A and B seem to have hit it off, leaving me in the dark about important subjects. I've been ignoring A recently due to her dependency on B because they're the best of friends now... A shouldn't need me if A has B, right? I'm getting tired of A because A only talks to me when A needs something or B isn't available. A's talked to me, time and time again about subjects concerning B. Honestly, I think A should just confront B already, but A told me that "it'd be a nuisance to B."

I also have another friend (C) who can't seem to do anything without someone by C's side.