Having best friend issues?

my very best friends and i have always been a group of three. it started out as my friend and i but another came along and we've been very close ever since grade 11. come university, we did drift a bit but we're still extremely close. now as of recently, maybe the middle of the summer, i have become increasingly irritated one of them and i'm not too sure what to do. whenever one wants all three of us to hang out i'm very hesitant because i dont like the one.shes very rude, cynical and shes always been like this but its gotten worse. i'm not too sure if i should tell anyone or just distance myself from the one? i'm quite confused because i dont want to ruin the friendship with the other and make things awkward. i'm supposed to go out this friday but supposedly the one i dont like is supposed to be tagging along and i'm not looking forward to it. this does seem very middle school but its unsettling that i used to love this girl to death and now i cringe when she texts me or i hear her name.