At least three of ten 2,000-pound Air Force bombs aboard a truck exploded and gouged a huge hole in Interstate 40 near Checotah, Okla., before dawn Sunday when they were ignited in a fire caused as the truck and a passenger car collided, Army officials said yesterday.

Clare Thomas, spokesman for the McAlester (Okla.) Army Ammunition Plant where the Mark 84 bombs were loaded into the private truck Saturday, said the explosives had been defused and unarmed but not protected against fire. She said the only defense against fire was a small fire extinguisher.

The same weapons produced for Navy bombers are coated with fire-resistant paint because they are kept on aircraft carriers that cannot be evacuated in fires, Thomas said. Air Force bombs are not coated because they are stored on land.

"It could have given them an extra margin of safety if they had the thermal coating," Thomas said.

Thomas said the Army Safety Command is investigating the blast that left a crater 40 feet wide and 25 feet deep, caused minor injuries to 42 persons, forced a predawn evacuation of 5,000 residents and shook buildings 25 miles away.

Army officials here described the explosion as rare and moved to reassure motorists by citing statistics showing only seven accidents and no explosions in 45,000 overland shipments of munitions in 1984.

"You take all the reasonable precautions but sometimes life deals from the bottom of the deck," said Maj. Phillip Soucy, an Army spokesman. "This was a normal 2,000-pound bomb shipment with the bombs loaded in the safest condition possible."

The Oklahoma highway patrol said the bombs exploded after a car entered the interstate from a road shoulder and collided with the truck. The car, pinned against a guardrail, caught fire; the fire quickly spread to the truck.

An Army team found that four other bombs apparently were destroyed in the explosion of the first three, Thomas said. The three unexploded weapons were taken to Fort Chaffee, Ark., for disposal.

Thomas said the 8-foot-long bombs, each containing 900 pounds of TNT, were loaded Saturday afternoon into the semi-trailer truck. McAlester contracts with private companies to ship weapons for the Navy and Air Force. The bombs were destined for the Sunny Point Terminal in Southport, N.C., before being shipped to a U.S. Air Force base in Spain.

She said the bombs were enclosed in shipping containers, blocked and braced into the truck with lumber and covered by a tarpaulin. Fuzes and detonators are shipped separately.

Because of the potential hazard caused by TNT-filled weapons, she said, McAlester is planning to shift its production to plastic-bonded explosives, which are fire resistant.

In 1971, three civilian employes of McAlester were killed in an explosion while neutralizing 20mm ammunition rounds, she said.