Trouble with friends

Okay, well I have these two really good friends at my new school.

One of them (Margret) was actually really close to me at the beginning of the year. We wouldnt stop hanging out with each other.

Now we hang out with a girl named Tara. At first I was fine with hanging out with her. But I had only one sleepover with Tara and Margret, and after that Tara and Margret seemed to be best friends. They have tons of sleepovers, and I'm never invited unless I ask them to (and I usually don't like that because I feel annoying or rude to invite myself).

And don't say to find another friend. There's only nine kids aside from me in my class. I'm friends with a few of them, but they still don't replace Margret and Tara. There's two other girls, but they're as close as Tara and Margret. And I'd feel weird being a 'best friend' with a guy.

I don't know what to do. I feel so left out. Help?

Id say, talk to them. Have lunch with them and maturely ask them why they have been hanging out and not inviting you when you invited them both for a sleep over and see what they say. Being honest and upfront is much appreciated and you will get the reasons from their mouths.