I mean technically speaking we don't know for sure if Fukami is into Kase.
Even if she is, (and she probably is), we don't really know if it's gonna become a full-fledged love triangle.
The arc could be about Fukami quietly being angst-ey and pining for Kase, all while Kase continues to be an oblivious dumbass about anything other than how thirsty she is for Yamada, all coming to a head with a "getting it over with" confession that a surprised Kase obviously rejects. (There could even be multiple confessions that all go over Kase's head until Fukami finally gets tired of her shit and says it as plainly as possible, that could be fun.)
I could see it developing to where Fukami ends up telling Kase that she likes her with one of those dumb "I know it's gross because we're both girls I'm sorry please don't hate me" things and Kase being like, "Well actually the person I'm dating is Yamada so you don't need to worry, we gucci."
My ideal scenario ends with Kase and Yamada playing wingwomen for Fukami, ending up with her getting a girlfriend and then they all go on awesome double dates. Bonus points if Fukami hooks up with Inoue.
...Who am I kidding? It's time to buckle in for several chapters of pointless love triangle drama.