I admire you, skater_girl! You have a question and you seek help online. Well done! Really, well done! You are miles ahead of your friends. Honestly speaking I didn't read your entire post (tl;dr), but I can tell you this: look up transactional analysis. Basically it explains *how* you interact with other people. Simple example: if an adult asks you a childish question; do you respond childish or as an adult? In your case: your friends are being childish here. How are you going to respond? Like a child or as an adult? Follow the two adult answers that were given before mine. And you won.
I don't think you need to stop being friends with Carlee just because the other 2 don't like her. You sound like a leader and not a follower. So continue to be friends with all three and like imanaddict said, talk with the other 2 and make sure they know that you feel differently about her and want to remain her friend and to not speak bad about her. Just one thing to remember, if they talk about her to you and they like to gossip, most likely they talk about you behind your back as well. It's human nature. I would try my hardest not to wind up in the middle of anything.
You need to have a talk with all 3 of them and let them know how you feel. If Carlee is your friend, you can simply tell the other 2 that you would appreciate them not talking about her in a negative way when you are with them, and vice versa. Your boyfriend is right about not getting caught up in the middle. That's a hard place to be and it is draining. There could be some jealousy going on with Darcie and Raevin and that could be the reason they don't like Carlee. Teen girls can be catty so just watch how you approach the subject, but a talk definitely needs to happen.