Am right to feel this way?

Ive been seeing this guy for about a month and things were getting serious. A couple friends of mine introduced him to me. Anyway few days ago when i was talking how serious we are getting to my friend boyfriend as they are really good friends, he casually told me that him, his girlfriend(my close friend) who is my really good friend and the guy was seeing been having threesome but not since we met.
The guy was seeing never told me and we talked alot about our past when we were bonding etc... I cant help but feeling betrayed and my trust violated. I am so upset with the guy was seeing and my friend (girl), not so much her bf because at least he told me when he realised how much serious i was getting with him!
I have short fuse and think the confrontation wont be pretty...
What would you do? I am starting to be fuming....