National Airlines yesterday returned to service the Boeing 727 jetliner that lost all three of its engines and 13,000 feet of altitude during a few tense moments over the Atlantic Ocean Friday morning. There were 110 persons on board at the time.

The reason all three engines "flamed out," or quit running, is still under investigation, according to spokesmen for both National Airlines and the Federal Aviation Administration. The three-man flight crew was able to restart the engines and make a routine landing at Jacksonville, Fla.

Mike Clark, a National spokesman, said the airline, the engine manufacturer and FAA experts had run extensive tests on the three engines and found everything normal. "We're still trying to come up with an answer," Clark said.

The engines - Patt and Whitney JT8Ds - are considered to be among the most reliable in the world, and are the standard power plants for such ubiquitous jet aircraft as the Boeing 727 and the McDonnell-Douglas twin-engine DC9.

Both JT8D engines of a Southern Airways DC9 flamed out in a ferocious rain and hailstorm northeast of Atlanta last April 3. The pilot attempted to crash land on a rural highway through New Hope, Ga., when he was unable to restart either engine.

Sixty persons were killed, including eight on the ground.

Extraordinary weather is largely blamed for that accident, however. The National flight Friday was in clear skies.

According to National and FAA officials, the Miami-to-Netward plane was over the Atlantic at 33,000 feet about 150 miles east-northeast of Orlando when the No. 1 (left) engine quit.

As flight engineer Dominic L. Fote turned on fuel heat (in case there had been icing) and a booster pump, the No. 2. then the No. 3 engines also quit.

The crew was able to restart the engines one at a time.

The first suspect in such an incident would be contaminated fuel. FAA spokesman Dennis Feldman said inspectors checked the fuel in the plane, its fuel filters, and in the two fuel supplies - at Fort Lauderdale and Miami - and found no evidence of contamination.