I am in the best friend's position at the moment. My best friend and this guy who I was very good friends with are "dating." I don't hang out with them anymore because I feel like I'm third wheeling. I only hang out with them if our other friend is there. Second, unfortunately, is the best friend may not like that you two are together too.
My best friend and her significant in my opinion are a terrible couple. He's kind of a douche so in small doses he's fine but whenever I'm there he doesn't treat her all that well and I can't be around them anymore unless there are other people there to buffer. I hope you're in the first boat and not the second.