this particular kind of poly relationship is called a hinge! where 2 people are in a relationship with one person but not each other. its not a particularly uncommon relationship type but I agree that the conflict resolution felt kinda meh. "its ok if they keep secrets bc I do too!!" what of.....none of yall did lol. honestly violating the terms of the relationship (having one on one sex when agreeing that they would only do it as a group) like that would've honestly been enough to call cheating in a way lol and he was right to be upset about that.

I've never been into this kind of relationships so I can't judge other than by my own biases, but the development for that throuple doesn't feel... Healthy?
Two of them are in love with the same guy, but not at all into each other. That arc with two of them fucking did not ease any of my feelings of uncertainty.
I felt betrayed when they broke the trust, then disgusted at the response, and the "conclusion" of the arc did not feel like It solved anything? It still feels like they'll forever be in that wishy-washy state that could blow up any second. As of now I legit feel it'd be better if two of them became a couple.
Maybe I'm just too close-minded for this.