I've been involved in many (MMF) three-ways. I love them. The first were with a buddy in the Navy. I was the better looking and better built, but he was the much better hung (7 - 7.5 ?). Typically, after a variety of foreplay, first I'd screw her, then him. The size difference didn't seem to make that much difference most the time, but often the chick would get off big time with him compared to me, sometimes even after I'd failed to get her off. Usually the girl was somewhat discreet regarding the size difference, but sometimes she'd comment on it quite openly and even make her preference for his bigger cock known. One chick, while playing with our cocks side by side, looked up and asked me if I'd known how much bigger my buddy was. I'll have to admit I got off big time on the size difference and loved watching him mount her with that big cock.
Later, when I was finishing up my last two years of college, I had a menage a trois relationship with a Jewish girl I'd been going with and an Italian-American guy who attended class with us. He was decent-looking, olive complexioned, lean but muscular, and had a fairly big, thick, uncut cock. Previously, sex with her and me had been okay, though somewhat hit-and-miss as to whether she'd get off or not. He got her off big time, much quicker and more explosively and, after the first time, he took over the main "heavy-duty" responsibilities.