In my view, triads represent no fewer than seven relationships, for in addition to each couple's relationship with each other, there is the triad's relationship...and then each individual's relationship with their own heart and mind.
Ignea, IME this is a good example of "one of those life things" where we know in our hearts that we have to take a stand for ourselves, witnessing to what we're really feeling, being true to our relationship with ourselves...even though our fears or intellect tell us that things could go very badly from a relationship perspective. That is to say, our relationship with one or more of the couples or the triad itself.
If you do find a way to talk about your evolving feelings, and if it goes as badly as you fear, then one thing you will know for sure is that this situation wasn't robust enough to sustain the kind of love you have to offer. That is very hard. Whenever I faced something like that, I always reached back to the fact that, while the external relationships were coming up painfully short of my desires/dreams/essential needs, my relationship with myself was strong. Cherishing that relationship above all things led me to finally find what I always wanted...and while it was a long and sometimes painful haul, and looking back I'm not sure how I did it, it brought me to a place that is so far beyond what I could have dreamed of, most days I walk around giggling like a goofball.