That's why communication bid important. Tell him your open to it but there are things your no OK with. When I had my first threesome with my ex. She didn't want me to penetrate her (female) friend. I said that it was fine and km return she was OK with he receiving a double Blowjob. They kissed did 69 together and I ducked my gf from behind while her friend licked her.

What I found was we enjoyed each other more and there other girl was there more as someone to add some sexual pleasure. It was the first time my ex had been with another girl. She loved the exercise and it was enough to satisfy her. So it might be just enough or you. Just set ground rules first