i dont usually post on boards but i wanted to weigh in on this topic from a little experience..
I think it CAN work and i think the best formula is male male female- where the two males have been long time best friends. No offense to girls (and feel free to correct me if i'm wrong) but they can tend to go through 'best friends' like they go through toilet paper...but when two guys are best friends they never get all dramatic and the bond lasts for ages. Plus they dont get as jealous.
My best friend and i have virtually the same taste in girls, so that's important too. When i met my future wife my bff was head over heels for her as well, and when she and i were engaged she admitted that she really liked him too. With me not being jealous at all of my bff and interested in where things could go, we had a couple of threesome encounters and it went awesome-everyone involved LOVED it.
My wife and i wanted to ask him to move in with us so that the two of them could pursue a deeper relationship and possibly fall in love with each other. Unfortunately he was in the military at the time and he was then getting shipped off for a while. We decided to hold off....but while he was gone he got lonely, met a girl, got her pregnant and decided to marry the girl.
After a while i told my bff that my wife was falling for him and he was really bummed. He loves his wife and their child of course, but my wife was both of our dream girl. He told me that he wished he knew because he would have been more patient knowing that my wife was waiting for him.
The three of us know it would have been awesome and long lasting. Lesson learned: sieze the day. Seriously.