Billy Smith and the Goblins - The book series

Three teenagers from our own world, outsider Billy Smith, strong-willed Lexi Aquino, and self-centered quarterback Kurt Novac, are transported to another world where they must fulfil an ancient prophecy by defending a besieged civilization of goblins. In the third book of the trilogy, the three teens, led by Billy, now the Goblin King, must defeat a mad elven wizard before she forces the goblins and their human enemies into a final fateful battle or everyone they care about will die.