Exboyfriend trouble... ):?

So I found Out my exboyfrinend(alex)had been cheating on me with one of my friends(sammy)and today another one of my friends(Lina) told me he was flirting with her. I think they are secretly going out and he's cheating on both of my friends and idk wat to do about it cuz I love both my friends so I dont want to tell them Alex is cheating on them but I feel like I have to cuz I want to protect them from him. He's a jerk and I want him to suffer the way I did so I really want to tell both girls. Lina was there for me when I broke up with Alex and Im over him now and I told her it wad fine for them to go out but I don't like it if she's keeping it a secret from me and denys it when I ask...what do I do?!?!?