A classmate gave me this toy when we were in elementary school. I just discovered that it is a “super dimensional battle carrier” called Grandbirth. It is the headquarters of Space Sheriff Sharivan and it has two modes: Battle Mothership and the humanoid Battle Birth Formation.
A video of it in action is here. For some reason, it is labeled “Bird Star-S.” The bottom shows that it was made in Japan by Bandai in 1983 and that Grandbirth in Japanese is Gurando Bāsu (グランドバース). The three tiny yellow ships are a nice touch.