Yuki Yuna is a Hero: The Washio Sumi Chapter

The great Shinju-sama has chosen three ladies to be heros and defend the planet from the foes known as Vertexes, who are destructive beings who pose a threat to its peace and security. Sumi Washio, Sonoko Nogi, and Gin Minowa were unsure when they would be summoned to service, so they passed the time by doing nothing. They are almost defeated, though, when a Vertex suddenly appears, and they find they have no idea how to fight as a group of magical females. By sheer force of will, they succeed in defeating their foe, but following the battle, the three resolve to strengthen their collaboration and combat skills.