Good lord, speak plainly.
That's more of a swingers thing than a poly thing.
In the vast majority of poly relationships, people are in separate relationships with each of their partners.
Triads (and quads, etc) are hard-mode. They certainly exist, but they take skill and effort from everyone involved, and so are significantly more rare. And of those, probably only a minority are going to be into the whole group-sex thing in the first place so... not very often.