My boyfriend of nearly two years cheated on me with one of my best friends.

As far as I know this is an isolated incident but I honestly have no idea. I have two best friends, it's always been the trio of us ever since we were 9 or so. About 2 years ago I began dating this guy. Ever since we have been in a seemingly amazing relationship. However I found out from my friend herself that he cheated on me with her, multiple times. My boyfriend wants me to forgive him. My friend wants me to forgive her. And my other best friend won't even talk to either of them. I seem to be the only one who doesn't know what to do. I don't know who to forgive, if anyone. I don't know how to make our friendship work now that I don't trust her. I don't know if I can trust that this is an isolated incident or if he's slept with others. And I don't know weather or not to take him back. But more importantly I don't know how or if our friendship with the three of us girls can be repaired. I need some advice.