What do you do when you start to lose your best friends?

Ok so I have 4 best friends. When we hang out we normally split into the same groups of 3 the majority of them time. I will just give you the names cuz it makes it easier to explain. Its always either me, Kate, and Allie or me Jenny and Sebrina. Well lately Kate and Allie have been leavin me out or making me feel like a third wheel. It really hurts cuz we used to all have so much fun together and now i feel like i dont belong in the group. So I started hangin with Jenny and Sebrina more and everything was fine for awhile until they started makin plans behind my back as to not include me. I dont get it. I havent done anything to any of them but yet i feel as if im losing all of my best friends. What should i do. Fix the problems or give them space? Or what cuz i want my friends back =(