I keep seeing people talk about polyamory route, but if they did it would be a seriously dysfunctional one. Suzuki clearly has no respect for either of them, treats Fumi as her "pet" and really doesn't give a shit about either's personal space, and Noa, respect for elders and wanting to make Fumi happy no matter what, still clearly isn't down with it. Indeed, even naive, overfriendly Fumi mainly seems to like having her around because the attention is flattering, which isn't sustainable.
A poly relationship between these three would fall apart in about a half hour.
Absolutely agreed. Their relationship is not functional as a poly couple. The senpai is clearly infringing on their relationship, not co-existing with them in any kind of positive way. There is no communication or mutual three-way feelings that are positive. Like others have said, Fumi is simply too naive, and Non-chan is kind of stuck dealing with the senpai trying to get all...friendly... with her girlfriend. It just doesn't work.