A three-alarm fire and the violent explosion that apparently touched it off destroyed a three-story garden apartment house last night in the Hybla Valley section of Fairfax County and displaced about 70 persons, authorities reported.

The blaze, which broke out shortly after 7 p.m., forced the evacuation of adjoining buildings in the Southern Manor development, which is south of Alexandria and just west of Rte. 1.

At least three persons suffered minor injuries. As of early this morning, fire and rescue personnel had not been able to make a thorough search of the rubble of the building at 7254 Fairchild Dr. because it had been declared unsafe to enter. Crews were working this morning to shore up the sagging walls and floors.

Officials said they had not been able to locate a resident, and did not know if the person was in the building when the blast and fire occurred.

No official information was available about the cause of the blaze in the three-story brick building. However, a Fairfax County fire official said that once it started it was fueled by natural gas.

"Everything just crashed down" when the blast or blasts occurred, said Laverne Smith, who lived on the third floor of the building that was destroyed.

"The windows blew out," she said, and "everything on the walls crashed down."

After hearing what sounded like two explosions, in succession, Smith said, she found the hallway of her building full of smoke.

Smith made her way out of the apartment by climbing over a balcony railing and lowering herself to the balcony of the apartment below, and her mother then handed down her 5-year-old son to her. The mother then made her escape through the smoky corridor.

The woman said she moved too quickly to notice the billowing fumes. "I didn't have time to choke," she said, "I was just gone."

"I thought the people up over me dropped a couch on the floor," said Cynthia Smith, who lives nearby.

"I thought the whole house was going to break," said Antoinette Miller, who lived some distance from the site of the blast and fire.

"The whole block shook," said another neighbor, Donna White. "I thought it was a bomb."

After the blast, witneses said, bricks and broken glass tumbled into the street, and flames leaped from the ground floor to the roof of the damaged building.

Witnesses said most of the brick wall at the rear of 7254 Fairchild was torn off, exposing the charred and burned interior of the building. Second story flooring appeared to sag ominously. Rubble was piled in heaps.

Other addresses in the garden apartment development shared a common roof and walls with the apartments destroyed at 7254. Exterior damage to those units appeared less severe.

Reports of an explosion and fire began reaching Fairfax County Fire Department headquarters shortly after 7 p.m., officials said.

The first alarm was sent at 7:24 p.m.; the second six minutes later, and the third two minutes after that. Special alarms were also sent, bringing the number of firefighters at the scene to 72. Using 25 pieces of equipment, they brought the blaze under control in 30 minutes.

Officials were preparing late last night to open Hybla Valley Elementary School, within a few yards of the site of the blaze, to accommodate the displaced residents.

Three years ago, a three-alarm fire broke out in the same 700-unit apartment complex, displacing 32 families. That blaze, which broke out Aug 15, 1981, and was believed to be of suspicious origin, caused about $600,000 in damage.