My wife and I are monogamous. But we do have one lifelong friend in a very successful marriage of two men and a woman. They keep it very low key and private. They are all mature and balanced people. It works for them, and their three children.
But I think that such relationships working is rare. It's hard enough for two people to stay in love and understand one another. Make it three, and also face social criticism - it would need to be very special people to pull it off.