Blue_83 saidAs a practical matter, it's often similar to some traditional forms of polygamy. The original couple is the primary relationship, and the third is essentially a secondary partner for one member of the primary couple. There may or may not be actual "threesomes" involved.
But a guy can dream, I guess.(j/k)
I was the 'third' in a 3-way relationship years ago; it lasted for about 6 months...I would say it was just regular hookups for a couple of months until they said "look, we consider you our boyfriend and we'd like to refer to this as a relationship".
My initial reaction was probably like most others - how would this work? I don't think I 'love' both of you, and I definitely don't like you both equally (sexually attracted to one over the other). But they were both really mature about it and at times philosophical, saying that it's not a triangular relationship - him and the bf had 8 years together so I could never be as important to either of them but why should it be that way, we all can relate to each other in different ways. The conversation went on back and forth over a few days.
Eventually, being a single guy with nothing really to lose and not looking for a one-on-one relationship at the time, I went ahead with it. Awkward at first I would say, but once we continued to communicate and all be honest, things were cool. The sex was pretty equal amongst all three - and the 'couple' didn't stop having sex 1-on-1 which I thought was pretty cool. It just made for hot times and, at the time, the attention and companionship, from TWO others, was great.
I had to move for work at the 6 month mark. Part of me thought "well where is this going to go long-term". I guess it could have gone on for years and years (like other relationships) but I had other opportunities to move on. They were both pretty upset actually; one of them found it hard to talk to me ever again. The both said they loved me.
I had never loved anyone at the time, so I'm not sure I returned that kind of intimacy.
So, as someone converted, I'm much more open-minded about other couples that choose to do this. It's not for me as a couple (in a LTR now for 5 years), but I think if you have any curiosity about it, there's no harm in exploring. The key would be that the 'third' doesn't become a replacement for love, sex and intimacy that a couple has together - that's what I saw worked well for the other two (who are still together, in a 1-on-1 relationship again). And, like any relationship, you must communicate open and honestly all the way through!
(Oh, and also prepare yourself from getting crap and/or judgmental comments from your friends).