How do I tell my friend that I don't like our mutual friend anymore?

Hi everyone, In my school I am somewhat of a loner - I have only one friend ("B") in my school. She has a few more friends, none she is particularly close with, other than another girl ("R"). R and I used to be very close for our first year of school. She was my only friend in first year, but she got very tiring very quickly. She's very high maintenance and I really just don't like being around her anymore. B joined our school in second year - she went to primary school with R, and I used to be very good friends with her but we lost touch. B being in the school brought R and I closer together for about 6 months - after that, she once again really started getting on my nerves. We are in sixth year now. I have been trying to end my friendship with R for the past four years. However, B still thinks that we are a trio and constantly tries to get us to hang out. Though I love being around B, and am willing to spend time with her, I can't stand R anymore. Whenever B and I make plans and she invites R, I quickly back out of the plans. The last time I hung out with R was about a year ago - B and I made plans to bake at my house, and she invited R. B had to cancel, so I also cancelled - but lo and behold, R shows up at my house anyway. We baked muffins - there were 12 total, so 6 each. There are 6 people in my family, 4 in hers. She said we should give B 2 muffins each, but I said no, as B wasn't even there to bake with us and I wouldn't have enough muffins for my own family. B then called me selfish and greedy. Today, B and R hung out together at B's house. (This doesn't bother me, I don't mind that B is friends with R.) Tomorrow there is a party - I made plans with B for her to come to my house beforehand to do our makeup, then come to my house for a sleepover after the party. However, B just texted me asking if R was also coming to my house - I said no, and B asked if I could talk on the phone. This is unusual - we don't really text or call eachother that much, so I knew it was about R. I told her I was busy, and asked her what was up. She said that R is worried about not having a lift to the party and B doesn't want to exclude her. B then asked if something was up with R and I. I was about to text her that R and I just aren't friends anymore, but started spiraling and had a panic attack because I'm worried that B will take this news badly, feel she has to pick between R and I. If this happens, she will pick R. I don't want to lie to her, and I really don't want to have a sleepover with R. B and R both have autism and I'm afraid that B will misread the situation. I really don't want to lose B, she's the kindest person I know and is one of my best (and only) friends. I'm going to call B tomorrow to tell her how I feel, but I don't know how to phrase it. I have pretty bad anxiety and I won't be able to go to the party by myself - the whole year is going, and I really don't want to miss out. I think that if I tell B, at the very least she won't want to come to the party with me. I'm thinking of telling B we can do our makeup together, then pick up R on our way to the party and drop her off on the way home - if I did this, we wouldn't be able to have the sleepover. But B lives an hour away from me, and we won't be leaving until about 12.30am - I don't think her parents will get her a lift home and I don't want her to get public transport by herself so late at night.

Just tell B that you don't enjoy R's company anymore. You'll still hang out with her if necessary, but you don't want R spending the night or hanging at your house. Honesty is best when discussing such things. Or you can just say nothing and deal with R when you have to.