I can basically understand why the primary focus of the arc was about how the three of them could be in a relationship at all, since that's probably the part most people would have the hardest time swallowing. It's just unfortunate that it came at the expense of fully establishing why any two of them have feelings for each other at all.
Nina and Amane were an established fact and perhaps didn't need building up, but they didn't really get a chance to showcase that supposedly long-standing relationship outside of equally long-standing inner conflict. I think they actually get one scene where they're happy and together, and even that culminates in Nina angsting quietly about Ryou.
Ryou apparently falls for Amane after a few pages of Amane being generically friendly while Ryou was injured, and she confesses pretty much immediately (at which point Amane doesn't even seem particularly interested in Ryou, as much as she just seems to like the idea of free love) but after that, most of their scenes together are either talking about polyamory or about Nina.
Nina and Ryou get the most screen time, but most of it is spent with Nina resenting Ryou. They have a prior relationship online, but they didn't seem to have much chemistry with each other beyond friends and fellow cat lovers, despite the joke page where Ryou's friend says it sounds like she's been hitting on "Shin'ya". They fight a bit, Ryou finds out that she's Shin'ya, Nina continues to be flustered and hostile while Ryou is positive, understanding, and inadvertently flirts a bit, and then suddenly Nina kisses Ryou, with Nina then realising that she's falling for her. They have some time offscreen, I guess, and by the time we get back to her, Nina's already talking about how much she needs and loves Ryou. Their relationship basically reverses completely in the space of maybe three scenes.
The way it all adds up, I'm just left feeling "eh" about the whole thing. I appreciate the effort to write a polyamorous relationship in a positive way, and trying to develop all three sides of their relationship in three chapters was definitely ambitious, but it didn't quite work out for me as a story.