The Adventures of Barry-no-guns, Cousin Tim and Dave (Terror from above Book 3)

Barry-no-guns, Cousin Tim and Dave are just teenagers who unwittingly find themselves embroiled in magic, bad people, demons, the undead, even aliens who are desperate to eliminate wizards and the human race. These adventures culminate with their travelling back in time to meet their mentor ‘Coagnus’.
You may think you have seen this before, but not with these three friends, without any knowledge of magic or the devious ways of international criminals and Galactic madmen, they have to overcome every adversary.
They rely on each other, they blunder from one crisis to another, but their friendship is their greatest strength, as for how they overcome their ordeals is explained by their simple statement. –‘We make it up as we go along’!