Kono Sankaku Love Come wa Shiawase ni Naru Gime ga Aru.

"What do you like about Reira so much?" Tenma Yashiro, an ordinary high school boy, accidentally discovers that Rinka, a cool beauty in his class, is in love with his childhood friend, Reira Tsubaki, a beautiful and innocent girl. Tenma then helps Rinka become friends with Reira, but... "If Yashiro-kun isn't dating Rinka-chan, then can I be your girlfriend instead?" "What did you do, Yashiro?" So Reira falls in love with Tenma, and a love triangle develops between him and the two most beautiful girls in school. What will happen in this complicated love story? Winner of the 28th Dengeki Novel Award Gold Prize! The happiest love triangle in the history of romantic comedy begins! (Source: Dengeki Bunko, translated)