Should I confront Ex girlfriend about possibly sleeping with my best friend.

I was visiting home for Christmas a couple of years ago and I'm pretty sure something happened between my girlfriend at the time and best friend at the time. Both me and my ex girlfriend are still in touch now as friends and me and the other guy are still friends too. What I suspect might have happened, happened 3 years ago. He stayed at her apartment the night, on the sofa apparently. I could tell when I was back in the country they were both acting very nervously around me and had both changed. After some time things turned back to normall between me and my girlfriend at the time but her and my friend went from getting on well to not talking to each other. We broke up a year ago due to geographical reasons, both of us came close to making the move but I ended up pulling the plug on it. I understand why they might have kept it to themselves because I was making a life in a new country and they were the 2 closest people to me.

I always felt the best thing to do was to let it go but with them both still in my life now. I feel I need to know to move on. Do I really want to know? Or will it ruin a very happy part of my life to look back on. Truth is I feel I won't be able to get over it if I don't find out exactly what happened. It's been 3 years now and still really bothers me from time to time. It would be easier to move on if it was an ex I have nothing to do with now and a random guy but that's not the case.