So called best friend playing us off against each other

So there are three of us who are all friends and work as nurses. One of them says to me I'm her best friend but then I find out she arranges to meet the other one and tries not to let me find out. 
If we are all on shift tigether she hates it if I'm alone taking to the other friend and wants to go to break with her ect all the time and not include me. 
It's getting silly I'm sure she's just jealous of something. When I wasn't looking she gave me a mucky look and the other friend asked her why I front of me! Why would she do that. 
Confused! And I'm fed up! We are grown women!! Not kids!!