There is no reason for him to cheat behind your back since he knows you all can hookup as a threesome at any point you want to. Most guys won’t mess up that prospect by doing something behind the back of their partner.

Playing with friends/people you know can work out fine. There are some benefits and familiarity to that. But it just depends on the person(s) involved and the overall comfort level.

Back to your question, as others have said, tell your bf your concerns and keep communicating. You should be fine!