Zillion

In 2387, Earth's most successful off-world colony is under siege by a savage race of warrior-cyborgs called "the Noza Empire". The Noza are seemingly invincible and the Marisian Defense Forces are severely outclassed. All hope seems lost when a trio of guns of immense power are found by the colonists and dubbed the "Zillion Weapon System" after the ancient civilization that created them and the substance "Zillium" that powers them. These guns are the only thing strong enough to destroy the Noza and are given to a team of three young colonists called "the White Naughts" whose members include J.J., Champ and Apple. They have been specially trained to use these weapons and are the last hope for survival for the Colonists.

Synopsis from Anime News Network