The Brave of Gold Goldran

Three elementary school boys, Takuya, Kazuki, and Dai stumble upon an ancient Power Stone while digging up an old time capsule. After being attacked by Prince Walter who also wants possession of the Power Stone they end up using it to summon an robot called Dran that is an ancient Brave that was sealed inside. Dran fights off Walters army then tells the boys that as his new masters they must help him find his other friends that have also been sealed in Power Stones that are scattered across the world.

Synopsis from Anime News Network