Three-way relationships are very hard; I remember I was part of a threesome from age 5-8 and we always inevitably sent one of us home in tears or two of us leaving (if we were at the home of the one who was "out"). In high school I had only two friends and they had been friends of each other from before I moved there but we were all so different that we didn't have that much trouble, went to our 40th high school reunion together just a few years ago.

Don't try to deal with your friends as part of a threesome. Enjoy each for whatever you enjoy them for and just "hang out" when you are together, don't worry about who is talking to whom or about whom, etc. and try to keep talk between two of you about the third (when you are one of the two) to a minimum, it does not help anything. Of course B is going to agree with you about A when she's with you! A would agree with your about B when you are with A if you had something to say. You are the same; you would agree with whichever friend you are with? That's very natural.

If A is not friendly, if you do not enjoy being with her then why are you her friend? Go out and meet someone else if you are tired of just these two friends; meet others who aren't "best" friends but are enjoyable to be around too.

If you sit near the guy in class, ask to borrow his notes and compliment him on them? Even if you have to pretend you want to compare them to what you wrote or that you don't understand some part, talk about the immediate class/problem. I did that once and got a date eventually. Talking about class things, notes, etc. will give you an opportunity to say something about other things perhaps. Start with what you know and ask his opinions and listen to them; find out what he's studying and what he likes and maybe even get his help with something if it's different from what you are good at. You can gradually get into music and other interests.