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.
Why don’t you tell us your background & context, the ones that make you ask this question?
I would like to explain that I’m not looking for dirty stories and such. I have been exploring the more extreme tropes in porn to see if they happen IRL or not. Probably should have explained better, but there it is. There are some ideas used that are rather extreme, and I start wondering how much it actually happens.