A flash flood swept three persons in a car to their deaths in San Antonio, Tex., yesterday, while the rampaging Pearl River bore down on the small city of Columbia, Miss., forcing thousands to flee.

Police in Houston searched for the body of a 14-year-old boy who disappeared in the churning waters of a bayou as he reached for a floating football.

Mississippi Civil Defense officials estimated flood damage statewide at nearly $700 million. In the state capital of Jackson, a waste water treatment plant was damaged $62 million worth, authorities said.

Some 3,500 to 4,000 persons abandoned their homes in the northern sections of Columbia, a city 8,000 in Marion County, and worried officials watched clouds darken. The Mississippi Highway Patrol said 6,000 people had evacuated their homes in the county.

In another flood locale, haggard volunteers shored up dikes on the banks of the Red River along the North Dakota-minnesota border. Grand Forks, N.D., and East Grand Forks, Minn., were hardest hit, with water a foot deep in East Grand Forks' business district and the river not expected to crest until tuesday. CAPTION: Picture, Grand Forks, N.D., resident brings more sandbags to a dike around his home. UPI