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Ok then, why three fins?  Well, the explanation starts with the big powerful engines.  They generate 3,250 hp each (twin row 18 cylinder 3350 cubic inch radials).  Huge props are required convert all that power to thrust.  With huge props, however, the plane has to sit high so that the props clear the ground.  A single fin would have simply made the plane too tall to fit into many of the existing airliner hangars of the time.  The solution was three smaller fins.