Well the sum of my experience has been to live in an MFM "V," two guys who are friends (the "arms of the V"), each guy having a romantic relationship with the same lady (the "hinge of the V"). We are poly-fi so we don't do the dating scene, it's just us three and probably always will be (although someone new joining us is technically possible). We've been together as a V since early in 2006.
Our first few years together were pretty rough. We had drama, and trouble figuring out how to live together and keep the jealousy and paranoia down to a dull roar. We had some good times, but a lot of bad times too.
But we hung in there, and after a few years things started to settle down. By now I'd say we've had a peaceful co-existence together since, oh, 2010.
Many things contributed to our successful cohesion. Lots of trial and error, figuring out each other's "tips and tricks," getting me on the right combo of meds (I've had various fun conditions, bipolar and others), living apart for a year or two, lots of talks and emails, figuring out our various needs and working towards meeting them as a team, making sure each dyad gets its own time and attention, etc.
Things are pretty boring around our house nowadays. Just the way we like it.