Romania's Bela Karoly, one of the world's leading trainers of gymnasts, who coached Nadia Comaneci to three gold metals at the 1976 Montreal Olympics, has defected to the United States.

Karoly, 38, joined by his wife, Marta, 38, and Geza Pozar, 31, the top choreographer for the Romanian national women's gymnastics team, informed Romanian officials of their decision to defect in a meeting yesterday at the State Department.

The Karolys left behind their 7-year-old daughter, and Pozar left his wife and infant daughter.

In an interview later, they said their decision not to return to Romania was made in New York on March 30, the last day of a four-week tour of the United States by the Romanian women's team.

The trainers said their decision stemmed from longstanding dissatisfaction with their treatment by the Romanian State Central Federation of Athletics. After Comaneci's spectacular success in Montreal, they said, she was treated as a national asset and removed from their tutelage for periods of as long as a year.

"This interference with Nadia's career led to her widely publicized decline between 1977 and 1978," Karoly said.

The three trainers, who have worked together for the last 11 years, said they hope to continue their careers in America.

Since her 1976 triumph in Montreal, where Comaneci, then 14, scored an unprecedented seven perfect scores of 10, the name Nadia has become synonymous with gymnastics excellence throughout the world.

In the United States, enrollment in girls' gymnastics schools has surged. Comaneci's picture was on the cover of Time, Newsweek and Sports Illustrated, and a record company sold a million recordings of the music to which she performed. One television soap opera even adopted the melody as its theme song.

Comaneci was only one of several world-class gymnasts trained by the Karoly team in the provincial Romanian town of Onesti, where the Karolys and Pozsar were physical education teachers. Therir defection is certain to be a serious blow to Romanian national pride, which received a great boost from the success of the gymnastics team.

Although their unhappiness with Romanian sports officialdom, highly centralized in Bucharest, had been nagging them since Montreal, the Karolys did not decide to defect until after a soul-searching conversation in the early morning hours of March 30.

The team was to leave at 7 o'clock that night for Bucharest. The three said they arose late that morning, packed their suitcases and left their hotel about noon without telling anyone of their plans.

While 15 other members of the delegation spent the day doing last-minute shopping in New York, the Karolys and Pozsar went to stay with a friend. At Kennedy airport, a Romanian national airliner waited three hours past the scheduled departure time, then took off without them.

Marta Karoly said they did not say any goodbyes because they did not want to have any of their friends held responsible for the defection.

"These were our girls, and we felt really sad," Bela Karoly said.

Their primary concern about defecting was having to leave their families in Romania, the three trainers said.

Pozsar leaves his wife, Maria, and his 18-month-old daughter, Karina Petra, behind. The Karolys have a 7-year-old daughter, Andrea, who they said is being cared for by Bela Karoly's aunt in Romania.

Yesterday, the defectors asked the Romanian government to permit their families to join them here. Romania's history of dealing with family unification after defections indicates that the Karolys and Pozsar can expect to be reunited with their relatives in about a year.

Their dissatisfaction with the athletics federation's interference in their gymnastics program began shortly after their triumph in Montreal when the government took Comaneci out of their school.

Karoly said the international titles won by his young gymnasts have been "seized on by the Romanian government as a vehicle to propel Romania into the limelight.

"The state left us alone until we were successful. But once we produced a superstar, they wanted to take her away from us. When Nadia slipped, the state rushed back to us, asking us to take her back. This exercise took place on three separate occasions, and it made us very unhappy."

On a tour of the United States in 1977, Comaneci was thought by many observers to be performing significantly below the standards she set in Montreal.

The Karolys said the government also removed from their school other star pupils -- Teodora Uneureanu, Emilia Eberle and Georceta Gabor, all internationally known gymnasts.

The Karolys and Pozsar are ethnic Hungarians, who comprise a minority of 2 1/2 million people in Romania. On the advice of their lawyers, Carol Critchlow and Alan Rothenberg, they refrained from a detailed description of their problems.

Last Thursday, they filed for political asylum with the U.S. Immigration and Naturalization Service. Their meeting yesterday with Romanian diplomats is a routine procedure for defectors from Eastern Europe.

From 1969 to 1978 in Onesti, a town of 30,000, the Karolys and Pozsar ran a small, but highly successful gymnastics program for local children, beginning at the age of 7.

All of their gymnasts, including Comaneci, were cultivated and trained by them, not sent to them by the government, they said.

In 1978, they moved their school to Deva, a town of 60,000 in Transylvania, and began working with a new generation of young gymnasts. "We left everything behind in Deva," Pozsar said.

Karoly said he would like to work as soon as possible in American women's gymnastics.

"We have a system that the three of us have developed," Karoly said. "We would like to offer that system here so that American women gymnasts can beat the Russians and the Chinese."