The author is really building up the concept of the three of them together, how they are the happiest when it's the three of them and how Ijichi and Amane both like each other just as much as they like Seo. Like when Ijichi for a second thinks Seo is voting for her alone in the beauty contest and feels somewhat uncomfortable at the thought of him choosing her over Amane. And also how whenever the topic of dating one of them, Seo can't even imagine choosing one over the other. This manga has a somewhat realistic tone and setting, and thus a poly outcome is kind of controversial, so I'm really hoping that the point of all this is to make this plausible in the reader's mind. Like, they all get together because it's the only thing that makes sense and makes everyone happy. Like, in most post-dating harems it's like the non-central partners tolerate each other more than anything. They'd rather have the central partner for themselves, but since that's not possible, they accept a harem because it's better than not being with them. But here it really feels like a poly ending is gradually built up to be the actual best outcome for everyone.