SAN JOSE, Costa Rica (AP) _ Police said Wednesday they think an international group helped the man who bombed a jungle news conference a year ago, killing three journalists and wounding Nicaraguan rebel leader Eden Pastora.

''All evidence we have indicates this man had the support of an international organization,'' Minor Calvo, head of the Costa Rican Office of Investigations, said in an interview.

''We know he did not leave Costa Rica through the airport,'' Calvo said, based on studies made of the handwriting of passengers leaving shortly after the explosion.

''We know he was using the stolen passport during that time and he used it to enter Costa Rica on May 5 of last year, but he did not use it to leave,'' Calvo said. Police said at the time that he could have gone by land south to Panama or north to Nicaragua.

''Finding this man will depend a lot on international help,'' Calvo said adding that many countries had been helping. ''All we have established so far is that he had a lot of support internationally to move around.''

Pastora, military leader of the Revolutionary Democratic Alliance fighting the leftist government of Nicaragua, called the news conference for May 30, 1984.

More than a score of foreign and Costa Rican correspondents from San Jose were taken by land and then by boat to a wooden shack on the Nicaraguan side of the border. The Costa Rican government considers the bombing under its jurisdiction even though it occurred across the frontier.

As the journalists crowded around Pastora a metal box supposedly loaded with photo equipment exploded, killing Linda Frazier of the Tico Times of San Jose, and Jorge Quiroz and Evelio Sequeira of Channel Six in San Jose. Pastora and a score of journalists were injured.

Mrs. Frazier's husband, Joseph, is a Central American correspondent for The Associated Press. Among those injured were Reid G. Miller, another AP correspondent in the region.

Investigations showed that the box was carried in by a man posing as a photographer and carrying a stolen Danish passport under the name of Per Anker Hansen.

Hansen was seen slipping out of the house just before the explosion, claiming to have suffered a few scratches. He returned to Costa Rican territory with the wounded and then went to San Jose, checked out of his hotel and disappeared.

Phonetics experts here and in Spain have determined after listening to tapes of the false Hansen's voice that, despite his fluent Spanish, he is ''a person of Scandinavian origin,'' Calvo said.

He said this has narrowed the search to northern Europe, and Costa Rican police are working closely with authorities in several countries.

Costa Rican officials have determined that Hansen, a tall man in his 30s with a short, dark beard, spent the two years before the bombing traveling in Central America with a woman companion.

Hansen posed as a photographer for a small French agency and made friends with Peter Torbiornsson, a Swedish television camerman. Torbiornsson introduced him to Pastora's rebel group and took him on a trip early in May to several rebel camps along the Nicaraguan border.

Pastora was a hero of the Sandinista revolution in Nicaragua, known as Commander Zero, but defected when the Sandinistas moved sharply to the political left after taking over the country in July 1979.