I'd consider obliging you Cheerios, except I don't feel like losing a ten-year relationship over it.
A co-worker of mine has stated that she is often in MFF trois. She lives with an unmarried couple that have a 1 year old, and says she wouldn't trade it for anything. The baby seems really happy, so I suspect there isn't a lot of emotional turbulence in the home.
Of course this is the exception and not the rule. I doubt most people could withstand the demands of a polyamorous relationship for more than a weekend, and the repercussions of an attempt would destroy a lot of currently stable relationships.
In other words, don't try this at home and don't try this with someone you can't afford (monetarily or emotionally) to lose.