Sound! Euphonium 3: The Greatest Crisis of the Kitauji High School Concert Band

Sound! Euphonium 3: The Greatest Crisis of Kitauji High School Concert Band (響け! ユーフォニアム3　北宇治高校吹奏楽部、最大の危機) is the third light novel of the Hibike! Euphonium series, and it was released on April 4, 2015.

Synopsis
The main focus is on Mamiko's past and her relationship with Kumiko as siblings. It also covers how Kumiko bonds with Reina, Shuuichi, Hazuki, Sapphire, Asuka, Natsuki, Mizore, Yuuko, Riko, Taki, and her sister Mamiko. It is these relationships that lead her to realize why she started playing music, and why she will to continue to play for as long as she can.

Major Events

 * It is revealed that Reina only came to Kitauji High School because of Taki-sensei, and that she loves him in a romantic way.
 * Shuuichi tries to grow closer to Kumiko and nearly confesses to her the night before their final concert. He also gives her a flower-shaped hairpin.
 * Reina confesses to Taki-sensei that she loves him after the final concert when all the bands/mentors are being honored, but he takes that as a of respect and honor rather than a sign of romantic interest.
 * Asuka mentions the hairpin on Kumiko after her graduation ceremony and asks if it's from her “boyfriend”. Kumiko stumbles and eventually says "Yes." This is meant to show that Kumiko has mutual feelings for Shuiichi.
 * Kumiko eventually confesses to Shuuichi that she loves him.