Well, my ex had a number of guys he had relationships with, but the only guy he ever had sex with was right toward the end, when he started shying away from our partnership due to various concerns. They were really more like unusually huggy mentoring relationships than anything, though. He'd help them financially sometimes, he would give any of them a place to stay if they needed it, we would go out and have a good time together, and it was pretty fun.
The way I remember it, though, if there are more than two partners involved, each one is going to have a different relationship. If you could say I was his "wife," they were "mistresses." They were generally very nice boys, and we got along pretty well. None of them were involved, though, for more than a year or two. My partner actively encouraged them to seek out long-term relationships with stable guys. When they would find someone, we'd take them out on dates together, we'd assist with moving, travel, the whole gamut.
Then again, I think that getting into serious romantic entanglements might have impaired our relationship. Right before we broke up, one guy started to move in, and he was okay at first with the idea of having three people involved. He freaked out pretty quickly though and bailed, and it was not long after that that my ex and I started talking about whether our somewhat proximity-impaired relationship was really going to continue working. When I started talking about getting moved in permanently and that became complicated, it seemed less and less like being together permanently was going to be realistic. The romance faded. Eventually, after our relationship had been entirely nominal for months, we ended up breaking it off. I went through the normal post-break-up depression and self-flagellation, got over it, and went back to business as usual. I still text him occasionally. He's doing great.
So the only thing I can tell you quite honestly works is for two permanently coupled guys to have a few casual partners. We weren't even having sex with most of those guys. Cuddling yes, dating yes, light petting and grooming pretty routinely, heavy petting occasionally. It's not that fucking them was out-of-the-question, but it just didn't happen. However, that might have had something to do with some of them being as young as 15 years old, so it would have been as illegal as hell anyway in those cases.
However, the guy I'm with now is so strait-laced that he would probably literally have a heart-attack or stroke if he found out I was so much as talking to another guy, so I don't think that all people are really compatible with polyamory. The irony is that he has two other guys who call him and talk to him for sometimes an hour each every night, and the one who calls up literally every night is sort of his ex. There is no romance between them, though, and my partner has really gotten to the point where he bitches about this guy more often than not. As he rants, I just sip my whiskey politely and nod.
"Yes, honey. You know I care about our buddy Jack too, honey. No, I don't think he's losing his mind, darling. Okay, honey. I understand, honey. Yes, I promise I'll handle him next time, honey. Dearie, you're getting out-of-breath; take a hit off your albuterol, would you?"
The other one calls somewhat less often. The funny thing is, anytime I have ever been in a relationship, I haven't had so much as a thought about another guy. I'm too introverted to do a lot of screwing around, honestly. It's not that I am against it as a matter of principle, but I just don't have the inclination.