Yuri no Aida ni Hasamareta Watashi ga, Ikioi de Futamata shiteshimatta Hanashi

Yuna Momose and Rinka Aiba were made for each other, and their whole school knows it. Between Yuna's princess-like elegance and Rinka's prince-like charm, all their classmates see them as the ideal couple. Yotsuba Hazama is no exception to that, but she is exceptional in another way: she's somehow managed to become fast friends with both of them! Having the whole school's favorite ship as her two best friends isn't exactly easy, though. Not only does it make everyone treat Yotsuba like a particularly obnoxious third wheel, it makes her feel like one too! Or at least it does, until one day, Yuna asks her out. And then Rinka asks her out too! And Yotsuba, whose social anxiety never seems to kick in until after she's already messed something up...says yes to both of them, without sparing so much as a single thought for the consequences. Oops! Now Yotsuba only has one choice: keep her accidental two-timing under wraps and make both of her new girlfriends as happy as she possibly can! (Source: J-Novel Club)