I'm currently in a triad relationship (yep, the unicorn) and it wasn't something I went searching for. I've known the couple I'm dating for over 3 years now... it maybe wasn't a natural evolution from friendship but the friendship helped us form a solid foundation.
I guess I also don't fall into the typical "unicorn" mold... I came out of a relationship but wasn't really in-between relationships (the previous relationship was more of a non-relationship) and I'm definitely financially and emotionally stable.
Oddly I'd probably be perfectly happy in a mono relationship too.