It started out with the third person asleep in the same room. I guess we were to loud, and he woke up. My partner didn't know we were being watched, it was a huge turn on.
Hmmm....I think I would let someone watch. I would find it thrilling because it would be almost like putting on a show for that third person. I also wouldn't mind watching two other people having sex. That would be erotic!
I had sex with a woman while her friend sat across the room and watched. She masturbated but never come over to join us. I put on an even better performance than usual just because I knew she was watching (even though I'd had sex with her also a couple of times before). It was fun. I'd do it again.
I had a girlfriend who ran with a bunch of gay chicks. One particular friend, slightly butch (maybe she had 'penis envy'), used to watch us go at it. We would perform to accomodate her requests. It was totally cool.
I used to have a regular MMF threesome. After a paricularly vigorous all night session, we opened the curtains the next morning and discovered a pack's worth of cigarette butts outside the window. Somebody had stood there most of the night! We were all pretty proud of ourselves!