I understand the snowballing downswing effect, but seeing as I'm the relative newcomer to a longer-term friendship with limited physical benefits between the guys here's how things usually play out in our polyfidelity scenario:
I get ticked at something E does or how he behaves as if he is not in a relationship with either of us. This is compounded in my head because it usually reminds me of something a previous ex did during a monogamous relationship that really ticked me off and became a game ender. Storm clouds descend.
T looks at the weather map of my head and gets me talking. We form a plan on how to get the three of us talking about the issue. I am reminded why T is so intensely wonderful and, by default, why E ain't so bad himself. Storm clouds blow over. Sometimes there's a rainbow, but most times I'm just happy there wasn't a tornado.
On occasion, it's me moderating between T and E. Very rarely does E moderate between T and me - that just isn't something he can do.
I think the toughest thing for me is that I sometimes think, "This is too hard, this three. I know T and I would be happy without all this damn work." But, in the end, we're happier with all of us in each other's lives. Sigh. Not much to be done about that.