MEXICO CITY : A Mexican admiral has been shot to death in an ambush in a region where security forces have been battling a powerful drug cartel, officials said Monday. The government’s spokesman on security matters, Eduardo Sanchez, said the military had arrested three members of the group suspected of carrying out the attack Sunday in the western state of Michoacan. He said gunmen opened fire on an official vehicle carrying Vice Admiral Carlos Miguel Salazar Ramonet, who was traveling on an unpaved road.