Iron Knights

Three best friends uncover a brooding secret in a deserted castle: what looks like old suits of armour are actually long-forgotten mechanical knights. They set about repairing them but their enthusiasm turns to panic when the Iron Knights turn out to be bonkers and almost completely evil. They reduce the boys' village to rubble and the friends are sent away by the rest of the villagers.
By luck, they are taken in by three separate Lords who train them to be knights themselves. They return a few years later to do the honourable thing: destroy the Iron Knights.