I've been in a closed triad for over 2 years. We never intended to get into the relationship, it just kind of....happened. Just like you none of us had done anything poly-ish in the past. My husband and I were married for 8 years and never did any swinging or anything.
We live together, share finances, etc. We definitely had some rough patches with jealousy in the beginning, but that has largely gone away with tons of communication. I've grown enormously in this relationship, especially with respect to owning my feelings.
There is nothing obvious about being in a triad. For us dealing with fallout from telling our respective families has been challenging. We have one child (from before the triad started) and are trying for another. I'm not looking forward to dealing with the extended family issues that will come from that!
Anyway, feel free to message me with any specific questions.