Do not allow your heroes to die. If you complete a level without letting any of your heroes die, you will earn three stars for that level. The amount of stars you earn has a minor effect on the gold you receive. However, earning three stars unlocks Blitz mode for that level. Blitz mode allows you to complete a level and get all possible drops without actually playing. This saves time later when you are looking for specific items. If you cannot get all three stars on your first attempt of a level, return later when your heroes are stronger.