CHERNOBYL, U.S.S.R. (AP) _ Three former high-ranking officials at the Chernobyl plant caused the world's worst nuclear accident through negligence, but did not abandon their posts in the crisis, officials said Thursday.

The three, who go on trial next month, reacted responsibly after the April 26, 1986 accident, said Alexander P. Kovalenko, director of information for the enterprise overseeing operations at the Chernobyl nuclear complex.

All three - the former plant director, chief engineer and his deputy - have been in prison while the accident was investigated.

Kovalenko said he believed each faces a maximum of 12 years in prison if convicted on criminal negligence charges.

Thirty-one people died and more than 200 others suffered acute radiation sickness after an explosion and fire tore through the plant's No. 4 reactor, authorities say. The government has judged 27 nearby towns uninhabitable for the foreseeable future.

Officials are conducting a massive effort to decontaminate an 18-mile danger zone surrounding the plant located 80 miles north of Kiev. Some areas of the zone are reported to have radiation levels just a few times higher than normal, well within safety limits, and have been repopulated.

This month's issue of the Soviet magazine Yunost reports that, before dealing with the disaster, top Communist Party and plant management officials fled the scene or went home to arrange their families' departure.

Asked by reporters on a visit to the town of Chernobyl whether the three defendants were among such officials, Kovalenko said no.

''They didn't run home to check on their wives and children and stayed on the job the whole time,'' he said.

The three are scheduled to go on trial on July 5 in this town 11 miles south of the plant.

Kovalenko described the Yunost article as exaggerated.

''He relied on rumors, not facts,'' Kovalenko said of the magazine's reporter. ''It was a very subjective article.''

The reporter, Yuri Shcherbak, quoted area residents as saying they called authorities shortly after the accident occurred and asked why people were not being told to keep their children indoors. He said the evacuation of some areas dragged on for eight days.

Kovalenko said the three defendants were responsible for the accident because they allowed unsanctioned experiments conducted by workers without sufficient training.

''They failed to educate and prepare the workers and six different safety procedures were violated,'' he said.

Soviet officials have said safety systems were shut off during the experiments, leading to an explosion and fire that spewed out radiation.

Kovalenko gave these descriptions of the defendants:

- Viktor P. Bryukhanov, 51, former director of Chernobyl's four-reactor plant, had worked there most of his career before his dismissal and arrest last June. A native of Tashkent, the capital of the Soviet Republic of Uzbekistan, Bryukhanov was a turbine engineer before rising to the station's top job. His wife, Valentina, still works at the station as an engineer in the industrial section.

- Nikolai M. Fomin, the former chief engineer, ''spent his 50th birthday in prison.'' Fomin, from the Donetsk area, was trained as an electrician.

- Anatoly S. Dyatlov was born in 1931 and worked as director of the No. 2 reactor before becoming Fomin's deputy.

The No. 1 and No. 2 reactors were not damaged in the accident and were put back into service last fall.

Plant officials say they expect to have reactor No. 3 on line late this year. Work on the station's nearly completed fifth and sixth reactors has been suspended at least until 1991.

The reactor destroyed in the accident has been entombed in concrete.