Proposed threesome with my ex and his best friend who i'm in love with

I was seeing this guy, Dom. He introduced me to his best friend, Liam, and we just hit it off insanely. I am completely in love with him but I can't act on it because of Dom. I broke up with Dom but him and me and Liam have recently become really close and I spend most weekends at their house. I share a bed with Dom, but we don't have sex any more, it's just like, it'd be weird if I didn't share with him when I stay round. Liam knows that we've broken up but it seems impossible for me to act on my love for him, and I know that he would never make a move on me because he is such good friends with Dom.

However, the other night when me and Dom were going to sleep, somehow the idea of a threesome came up. It's been hinted at before, what with us all being attractive and close and spending so much time together, it almost feels natural. I kind of drunkenly joked that I'd like to have one with Liam (not being serious) and Dom went straight to Liam the next morning and asked him! And Liam said he'd be up for that.

Having a threesome with two good looking guys who are close friends has always kind of been a fantasy of mine but is this a good idea? I feel that if I go through with this, I'll never be able to have a chance at a real relationship with Liam, but conversely, as that may not be possible anyway, I'd rather sleep with Liam under these circumstances than never sleep with him at all.