SAIGON, April 28, 1975 (UPI) - Gen. Duong Van (Big) Minh took over Monday as South Vietnam's third president in a week but failed in his initial plea to the Communists for an immediate cease-fire. Minutes later warplanes bombed Saigon's airport for the fi The leader of the coup that overthrew President Ngo Dinh Diem 12 years ago took the oath of office to succeed Tran Van Huong. Minh addressed himself to the Communists: "Let us sit together and negotiate and work out a solution." In Paris, a