...I think one thing that throws many couples for a loop when trying to form a triad is that their relationship will need to change too. If you want something long term with a third person, all relationships must evolve, including the original relationship.
I agree with peabean, having been one of those people trying to fit in with an established couple. In our case, it wasn't a triad per se. He was the hinge. He and I were heterosexual and she was bi. She and I were friends, but nothing more.
I had never heard of polyamory prior to meeting him. As I look back, I realize that I was the one doing most of the changing and adapting to the situation. I tried to be flexible, because they were raising her 8-year old son, and someone's childhood shouldn't be put on hold for adult relationships.
Now I can clearly see, however, that they did very little to change their relationship dynamic to try and accommodate me and my wants/needs. As long as I was willing/able to go along and fit in with their plans, their home, their friends and family, I was welcome to join them.
I joined them in their extended family events, and couples events. He and I would take her son along with us to do things when she had to work. I even watched her son one day when they both had to work.
But I wasn't included in the decision-making process prior to any plans being made. When I brought this to their attention, all of a sudden I was being too needy and jealous... and our relationship wasn't "fun" anymore (according to them).
I left the relationship, and I honestly don't think I was missed much at all. I would be very hesitant to try another poly relationship. It's too much work from where I stand.