As to why these three songs specifically, it is because these songs represent the current members of the group and the Kamen Riders they represent, namely: Hitomi Isaka (OOO), Chisato Akita (Wizard), and Tomomi Jiena Sumi (Gaim). All of these songs are arranged by the girls themselves. The album is set for a May 20th release.
[Source: Kamen Rider GIRLS Twitter, Heisei Kamen Rider Twitter]