Is it normal to not want to be best friends with this girl

A little (a lot) of backstory.... So I have a best friend(let's call her A) who has been since 2007 our freshman year of high school. She is now 20 & I'm 21. She had made a best friend(lets call her B) who is 19 for about a year from her job. I think they're relationship is so strong and cute! However, I met B a few months ago while visiting A at her job. A few weeks after us meeting B announced in front of all of her coworkers that we are now "best friends." Call me old school but I feel it takes waaaaay longer than a few weeks to become best friends especially being 21. I smiled and waved because I didn't want to embarrass her. I feel bad for drawing this out as long as I have. I've tried dropping hints(one being not responding in group chat) to both of them that I am not feeling the whole idea of us being a "best friend trio" which has recently developed but I don't think they caught it. A who served as the middle ground of the "trio" will be leaving for school in a week and I know that I will not be as great of a best friend to B as A has been because I don't know her that well and I to be honest I don't want to. I look at B as just a friend but have led her on to think it's more. How do I lightly break to B(and A) that I am not the best friend she thought and do not want to be nor sees her as such? I respect their individual best friend relationship and sometimes I feel B is drawing A away from me. I've never been in anything like this so I apologize for rambling! Thanks