Good god, 50% are looking for unicorns? Wow. That's... crazy. Put me in the camp of the stumbled-into rather than sought out, mostly because I can't imagine how one SEEKS a third person. I just don't get it.
Responding (sorry, no quotes) to an earlier point, I wasn't meaning to highlight the secret stash of successful triads as something special. I think there's an awful lot of polyamory that doesn't get announced as such, mostly in the form of V-type relationships with a member of a couple having another relationship, an "acnowledged affair" a friend of mine called it. I didn't mean to imply that triads are unique this way, though. Sorry about the confusion.