The third season doesn`t have a particular story behind everything (like Valkyrie Ghost in the second season). It just tells 6 stories about the life of Kazuto & Co. with some additions: Raine and Chorus get a house on earth, the last three Valhalla princesses each get an episode, and in the last episode Kazuto and Valkyrie, who in the same episode managed to stay adult, >try< to marry again. (Source: ANN)

DVD specials.

Koto unknowingly seals away the Sun Goddess Amaterasu, and now she is in a different world and can't return. Meeting priestesses who combat the evil in this strange place she learns that Black Towers throughout the land keep Amaterasu sealed away and are guarded by evil monsters. Koto must now find a way to help defeat these monsters and return the world to the way it was. (Source: ANN)

Episodes 5.5, 5.75, and 13 bundled with the second, third, and fourth Blu-ray volumes of ViVid Strike!