MARDAN- Armed men gunned down three persons including father and two sons while injured a passerby in firing over old rivalry today. Police said that the incident took place in Umerabad locality of Mardan where armed accused ambushed members of rival group. As a result of firing a man and his two sons were killed while a passerby sustained wounds who was rushed to local hospital for treatment. Police said that the triple murder was outcome of personal enmity. The police have registered a case against the assailants and started raids for their arrest.