Honestly? I don't think these girls sound like very good friends. While they may be good alone, they don't sound nice together. They sound like they mock you a lot, and that they might not exactly like you. But you don't HAVE to be their friend. I think you should make new friends, because these people just sound mean and rude.
By the sounds of it, you obviously haven't done anything wrong, it's just that these girls are just personality changing. What I suggest is to just stop talking to them. Get some new friends or people to talk to, and if one of those girls wants to talk to you individually, then try and avoid it. While she may seem all apologetic when she is alone, remember what she'll be like when she's with the other girl.
I've been in the same situation, but it was a little different (In my situation, I had boy X, girl Y, and girl Z. Box X would be very nice to me when girls Y and Z weren't around, but when they were around, he would make fun of me to impress them. Needless to say, I have not talked to Boy X since I noticed that he was doing this)
I hope you find some friends worthy of your friendship
~Stella