Yuuki Yuuna is a Hero: Washio Sumi's Chapter 3 - Promise

結城友奈は勇者である -鷲尾須美の章- 第3章 「やくそく」

Also known as Yuuki Yuuna wa Yuusha de Aru: Washio Sumi no Shou Movie 3 - Yakusoku

In the year 298 of the Divine Era, elementary school girls Washio Sumi, Nogi Sonoko, and Minowa Gin are tasked with an important mission. They are to become heroes and fight Vertex, a mysterious enemy that is attacking Shinju-sama, the god tree that protects Shikoku, the only area in the world that is still habitable. What they don't know, is that this fight will cost them more than they could have ever imagined.