Robin Bennett 
Iron Knights [PDF ebook] 
Putting the Evil back into Medieval

Three best friends uncover a brooding secret in a deserted castle: 3 huge 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 boysi village to rubble and the friends are exiled. By luck, they are taken in by 3 separate Lords who look after them. They return a few years later to do the honourable thing: become warriors themselves and destroy the Iron Knights.