archway asked, so I'm answering.
Probably the best thing about being part of the Trinity is the geometric growth of benefits. I believe it's easier to us to find X/Y/Z like/want/need in this grouping that it would be in virtually any ordinary pairing. If my other thirds want spicy foods, they know they can find someone who wants that, without badgering the partner (me) who doesn't dig on that sort of thing.
featherynscale and I can go watch zombie films, leaving
kittenpants to happily do something that doesn't involve brains in such a detailed manner. Somebody has an idea that I think is completely unworkable? They can go work on it together until it's a better idea, or until they've run it into the ground of their own volition. Sometimes I hear about it again, sometimes not... either way, nobody got thwarted or had to look very far for validation or a co-conspirator. This is a fairly big deal, to me at least, because I tried for years to fulfill the "one true destined love" fantasy
*, and mostly just made myself and my partners crazy.
Finances and chores scale to a large degree too, of course (not including
kittenpants' tendency to cook for either 1 or the Mongol Horde). But that's more of a side-benefit, even though we probably never would have gotten such a nice house without it (and we actually, almost, have enough room for all of our stuff here).
Sure, there's additional problems of expansion in size; triangulation, communication difficulties, imbalance/jealousy, but they're not as significant (for us) as the easy way in which we became, essentially, "more than the sum of our parts".
*
Yes, even while being in love with three different women at once. I don't get it, either...