On the episode "Spin", Wilson sagte to Cameron "I always told THEM (my wives)" speaking about the cheating, therefore I'm thinking he cheated on Mehr than one of his wives.
We know his third wife cheated on him, but did he cheat on her? I don't think so. Wilson sagte to House that he wasn't cheating on her on the episode "Sex Kills", and yes, I realize that Wilson could have easily been lying, but I can't imagine House not finding out about it eventually oder this not being shown/hinted to us (the viewers) at some point if it had actually happened.
So my conclussion is that Wilson cheated on his two first wives and the third one cheated on him. Karma is a bitch, Wilson x)