It's a triad. At least, that's what a threesome I knew called themselves. They even had children and both "husbands" went into the delivery room. Eventually, jealousy caused one of the men to leave. The other two remained together.
I also know those who are into polyamory (if that's how it's spelled). So that is sleeping with whomever ad infinitum—basically open relationship. But it's qualified by having relationships rather than just sex. It seems to me that it would be more difficult to trust, since humans just aren't that honest or transparent yet in general. You would have to be able to trust all involved, not just your original partner I imagine.
I also know those who are into polyamory (if that's how it's spelled). So that is sleeping with whomever ad infinitum—basically open relationship. But it's qualified by having relationships rather than just sex. It seems to me that it would be more difficult to trust, since humans just aren't that honest or transparent yet in general. You would have to be able to trust all involved, not just your original partner I imagine.