I think a threesome is really exciting when it’s in the context of a new person coming into an existing long-term relationship that may have grown a bit stagnant. When you are having pretty routine sex with the same person for several years, an injection of someone new is really, really hot. My first threesome was with my ex-husband, who I’d been with for quite awhile, and his girlfriend, who became my girlfriend. It was really fun, not awkward. My girlfriend was really experienced in both poly and in threesomes so I’m sure that helped.
Being the third person coming into an existing relationship doesn’t have the same *kapow* of excitement. It’s still awesome – sex with two people who you really want to sleep with, yay! And it does still have the element of exhibitionism/voyeuris m that can be a big turn-on, if that’s your thing.
I’m in an MFF triad now, and have been for over a year, although it’s long distance. At this point threesomes are a regular part of our sex life. When I’m visiting them, we have threesomes roughly as often as we have partnered sex. So it’s become a lot more ordinary.