Three black nationalist insurgents, all of them members of the banned African National Congress guerrilla movement, were condemned to death today after being found guilty of high treason yesterday by a Pretoria supreme court.

One of the men, David Moise, 25, was found guilty of helping other insurgents lay mines on South Africa's strategic oil-from-coal manufacturing complex of Sasol on June 2, 1980. The resulting explosions caused $7 million in damage in the country's worst case of industrial sabotage.

The three men entered court today, as they have throughout the long trial, singing freedom songs. When Judge Charl Theron imposed the sentence, Moise and Johannes Shabangu, 26, smiled. Moise turned to the gallery to give a black power salute, which was returned by his mother, the South African Press Agency reported.

The third man is Anthony Bobby Tsotsobe, 25.

All three admitted being members of the Congress, which last week claimed responsibility for the rocket attack on Voortrekker-Hoogte, a major Army base in Pretoria. They left the country during nationwide black unrest against the white minority government here and received military training in Angola.

Today's sentences bring to eight the number of people condemned to death for Congress-related insurgency activities, which have become more frequent and sophisticated in the past two years.

The high treason convictions resulted from the men's admission of membership in the Congress and of participation in terroristic activities. There were no deaths as a result of any of those incidents. Tsotsobe was found guilty of launching an attack on the Booysens police station in Johannesburg April 14, 1980, with a rocket launcher and Soviet-made rifles. He was also found guilty of planting bombs on a railway line in Soweto and of attempting to burn down a hall in Soweto.

Shabangu was convicted of throwing a hand grenade at the home of a black policeman and of having a "death list" of seven blacks to be assassinated because they worked against the Congress.

Most of the state's case rested on signed confessions from the defendants made while they were in police custody without legal representation. During the trial they testified they had been tortured into making the statements, but Theron ruled the statements were admissable.

"There has been a marked increase in terrorist acts and for the moment I am disregarding them," Theron said, reading his sentence. "I have searched my mind to think if there can be anything mitigating enough that could prevent me from imposing the death sentence.

"They must have realized that in the event of the deeds they were about to commit and did commit, being successful, the deaths of innocent people would be inevitable."

A defense lawyer said that Theron will hear arguments on whether to allow an appeal.

Only one of the five other condemned men has been hanged. Solomon Mhlangu was executed for his part in the deaths of three white men in Johannesburg, although he did not actually shoot anyone. Another, James Mange, had his sentence commuted to life imprisonment in 1980 and three others are awaiting the outcome of appeals.