A bomb planted by suspected communist rebels exploded in southern Nepal on Wednesday, killing three policemen and a civilian, an official said. Five policemen also were injured in the explosion at Janakpur, about 150 miles southeast of Katmandu, a police officer reached by telephone said on customary condition of anonymity. The bomb exploded when the policemen were trying to remove sacks left by the rebels to block the roads. The bomb had been hidden behind the sacks.