Aniplex announced on Saturday that Aimer will perform the theme song "Haru wa Yuku" for Fate/stay night: Heaven's Feel III. spring song, the third film in the Fate/stay night: Heaven's Feel film trilogy. Aimer also performed the theme songs for the first two films, and Yuki Kajiura composed and produced all three films' theme songs. "Haru wa Yuku" will ship as Aimer's 18th single on March 25.
The film will open in Japan in spring 2020. Tomonori Sudō (Fate/stay night: Unlimited Blade Works, Fate/Zero animation director) is directing the films, and ufotable is animating the work. Yuki Kajiura (Fate/stay night: Unlimited Blade Works theme song composer, Fate/Zero music and theme song composer) is composing the music. Takahiro Miura is providing the storyboards.
The first film opened in Japan in October 2017, and the second film opened in January.
Sources: Press release