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)

The strange and magical events of the last season have left Kazuto with two Valkyries. He now finds himself living with and looking after both a teenaged-princess Valkyrie, as well as the child-princess Valkyrie! And they still have not yet gotten married... (Source: AniDB)

Valkyrie, Kazuto, Akina, Hydra, and friends are back for more when the Key of time suddenly appears along with many other mysterious happenings. All seemed to be linked to a person named Valkyrie Ghost whose only goal is stealing Kazuto for herself. As they team up against this seeming unbeatable foe they dig deeper into the mysterious past of Valkyrie's home planet, Valhalla. (Source: ANN)

UFO Princess Warukyure, aka UFO Princess Walkyrie is about a princess from outer space who accidentally crashes on earth, where Kazuto desperately tries to maintain the public bath of his grandfather. Due to circumstances, Kazuto receives part of princess Walkyrie's soul which forces her to stay there with him. But that's not the only problem ... because her soul lost strength, the princess transforms both mentally and physically into a little kid! (Source: AniDB)