Friendship problems, help?

So i have two friends, and in class we're known as the best friend trio. I'll be referring to them here as L and D. I was best friends with L for a year before we became friends with D; so I know L longer is friends with her more. Now, D just came into our friendship, and we let her join in...so she was our friend. But now, she keeps on being bossy, like telling us what to do, and I don't mean to sound mean, but she's turning bitchy. She gets mad when I get better grades when her when it's not my fault- isn't her grades her responsibility? Then me, being the stubborn and hot tempered person I am, would always argue with her. Then when I get a grade that isn't failing, but something below what I usually get (90-100), she would laugh and rub it in my face. When I talk to L about something she would always interrupt and get L's attention (and no, not like flirting attention lol, they're both girls) and then she would try to get in the convo. I had a fight a few months ago because she got mad that I got better grades than her and she felt that I was competitive when I'm not. True it is that I try to get good grades, but if I don't then I just accept it and try to get a higher grade. L was trying to stay neutral, but after a week or so, she confessed to me that she was on my side but didn't want to hurt D's feelings. D is basically, becoming a totally different person. When I talk with my other friends she gets mad, like possessive almost. When I play cards in class (since it's summer, we barely do learning in class now) she gets mad that I play with other people, but well, I have other friends. L understands that I have a social life and hang out with other people but D just wants me to hang out with her and her only. What should I do?