Are my friends just using me for when they are bored?

OK well my bff marie(not her real name) and my other bff kim(not real name) and me are always hanging out. But sometimes I wont be doing anything and i will invite one of them over but they say there busy. But later i find out they are at each others house everyday and im just the 3rd wheel. What do i do to stop this because i have been friends with both of them since kindergarden and we are now in 8th grade. And they dont just do that but everytime kim is at maries house or marie is at kims house they always call me and rub it in and start getting mean and cussing me out for no reason if i get mad. And they even say there rubbing it in. What do i do? PLease helpp !!!!!!!!!!!!!!!! :)