When I was in a triad, we had to learn that if person A and B were having an issue, do not bring person C into it. If it were a minor disagreement, person C may be useful to help bridge preception problems or by helping with suggestions. But if it got heated at all, person C should back out and A and B should not try to pull that person back in.
But like all good life lessons, it sometimes takes several tries before I relearn them. My wife's best friend stayed with us for several months. I tried to help out in an argument between them and realized why the rule was made way back in my former triad.
I remember once getting into a fight with my wife and she said, "let's get another opinion." He friend suddenly gave us a paniced look. It was actually pretty funny.