Yuuki yuuna is a hero: washio sumi's chapter 3 promise (2017)

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.