Stichwort Eldran Series

The Kikaika (literally mechanization) Empire invades our solar system, and in a matter of moments they take over and convert Neptune, Uranus, Saturn, Jupiter, and Mars, and set their sights on Earth. However, one thing stands in their way. Earth's ancient protector and soldier of light, Eldran. To fight this menace he gifts a 5th-grade class with 3 huge robotic dinosaurs, with the power to merge and form the giant robot Go-Saurer, which they then can pilot and use to stop the Kikaika Empire's advances.

Three boys—Kotaro, a ninja in training, Rikiya, a training baseball player and Yosuke, a studying scholar—were living normal lives in Aozora Village. One day, Kotaro, along with his father Tobe, accidentally broke the seal that links the Underworld from the Human World. Eldran, the warrior of light, appeared in front of Kotaro, Rikiya and Yosuke along with three strange machines. They were appointed to save the world from destruction as the Ganba Team and use the robot Ganbaruger.

Elementary school children take control of giant robots to save the earth from the evil Jaku Empire.

This OVA is a sequel to the TV Series 'Matchless Raijin-Oh' consisting of 3 episodes.