As much as we didn't saw many interactions - and I mean I think we only saw Fushimi kissing Komichi and never an interaction with the boy - which could lead us to think he's just there as the third wheel, the lack of information could also mean that in reality, he's living the dream....
I'll be honest, since the "let's be a throuple", it does feel like the author didn't know how to move forward anymore and the lack of kissing between him and Fushimi (i don't remember if he kissed Komichi, if not it only strengthens my idea) makes me think the throuple thing was forced on him and author would have much preferred to do a GL story....
But, tbh, I'm just content with the fact that there's people starting to work on the idea of throuples....
All this just to say that the way you're looking at this feels more like a love triangle than a throuple...
Sure, in reality, it's not something feasible, looking at how much we're tied to the concept of monogamy by centuries of religion and whatnot.
Anyway, point is, while difficult, it can be possible to find 3 open minded people that all love each other and nobody is excluded.....much like we accept homosexual, transgender or more kinky couples.
What you're describing, feels to me like "a cuck, a hoe and the third element" (or the gender reverse of all that)......which, again, is not a throuple.....