PALMDALE, Calif. (AP) _ Three air traffic controllers who used illegal drugs while off duty must undergo rehabilitation at their own expense or face dismissal, a federal official says.

The drugs traced in urinanalysis were found to be amphetamines, cocaine and marijuana, said the official, Don Early, manager of the Los Angeles Air Traffic Control Center here.

Fifteen other controllers have been cleared and allowed to return to work, while investigations continued for 16 and for five center employees who are not controllers, Early told a news conference Tuesday.

''The Federal Aviation Administration is moving ahead carefully with the investigation to protect the air traffic control system and those controllers who are innocent,'' Early said.

The investigation stemmed from a Los Angeles County Sheriff's Department search of the home of center secretary Karen McIntosh on July 8. Investigators found papers that led them to suspect drug use among controllers, Deputy Chris Robbins said. Mrs. McIntosh, 27, and her husband, Steve, 30, await trial on drug possession charges.

The FAA began its own investigation, resulting in the removal of 34 controllers from their radar scopes, which monitor an area from Bakersfield, 225 miles south to the Mexican border, and approaches from Utah and Nevada.

The controllers had been placed on administrative leave Aug. 21.

Investigators never alleged there was any drug use while on duty, Early said.

Some of those under investigation complained their reputations were ruined by the investigations even though they were never charged with a crime.

''There is still a chance that my career has been destroyed,'' said Dennis Cottle, a five-year veteran who was among those cleared and was scheduled to return to work today. ''It takes an edge to do what we do. We face the possibility of a mid-air (collision) every day and it doesn't take much to destroy that edge.''

Early defended the investigation.

''Removal of these individuals from active duty while the investigation is taking place is in no way a reflection on them as people or on their ability to progress in their careers.''

Air safety has not been compromised before or during the investigation, Early said. He said controllers at Palmdale were not guiding the aircraft that collided Sunday over Cerritos.