Thanks for your input, Pollyshari!
I can see a triad working when A, B and C relate in following ways; A is friends with B, B is friends with C, A is friends with C. In my ABC-friendship triad, usually when any two people get together, talk at some point turns into gossiping about the absent friend, making gentle fun of them and then wondering why they are not there and if we could maybe call or drive by their place. Or we can always have a conference call, if they are REALLY busy.
I could see the power balance in a triad going three-ways (at least):
1) married people (or the original couple) first. The couple has known each other longer and is used to backing each other up.
2) gender solidarity. In most triads, they are two people who share a gender identification and the acculturation that comes with it.
3) the more connected couple first. Usually it's somewhat hard to talk, have sex or spend time together with both of your two partners equally. The one whom you feel more connected to might change over time and from situation to situation, though. Say A and C are sports fans and B detests sweating, even if by watching others do it. B might have to think up something to do during Super Bowl season.
All of these 3 possible power pairings are likely to be at work at the same time, and if they are well-balanced, I don't see why the situation should be necessarily exploitative/secondary for the unicorn.
Me: bi female in my twenties