Three young men were killed and three more critically wounded outside Cape Town today when police, hidden in freight containers on the back of a truck, opened fire with shotguns on about 150 stone-throwing young people.

Residents of Thornton Road in the Colored, or mixed-race, suburb of Athlone said the neighborhood had been quiet until the truck, owned by the state railways and commandeered by the police, drove through the neighborhood. It made a first slow passage down the street, turned around to make its way back, and, drawing a volley of stones from youths, then turned around for yet a third trip, this time with the police firing shotguns at the crowd.

The residents said at least six persons were killed and more than a score of others were wounded, some of them as young as 8 years old.

Although the police said the shotguns were loaded with birdshot, meant to sting but not kill, Thornton Road residents said some young men were hit with the full force of the blast at almost point-blank range. Residents said that some of those wounded had been shot through the windows of their homes and that soldiers had kicked down doors to seize those fleeing.

A police spokesman in Pretoria, acknowledging the "Trojan horse" tactics, confirmed that policemen had "commandeered and manned" the truck in an effort to halt the "repeated incidents of stone-throwing and barricading of roads" in the Athlone area.

"We have all sorts of little strategies," the spokesman said, refusing under police policy to be quoted by name. But "they would not have been shot if they were not throwing stones," he said. The police said they arrested 10 youths.

Earlier, a black man was burned to death, according to his family and neighbors, when policemen fired two tear-gas shells into his house at the KTC squatter settlement outside Cape Town, setting it on fire and apparently suffocating him as he slept.

"The police fired a tear-gas bomb into the house, and it exploded when one of the boys threw water on it to put it out," Nonvuyo Mdlanghathi, 22, the widow of Leonard Mdlanghathi, 35, a security guard, told reporters later. "Others took all the children out. While we tried to put the fire out, another tear-gas bomb came through. My husband was suffocated by the tear gas, and I was trying to pull him out when I started to suffocate, too. I was pulled out of the house by a neighbor. I had to leave my husband, and he burned to death."

The police replied that they had fired tear gas only after the house had begun to burn in order to control the crowd and allow firemen to get to the blaze.

In Pretoria, a black poet and avowed supporter of the outlawed African National Congress, condemned to hang in the 1982 murder of a security policeman, lost his appeal for a new trial or a review of his death sentence when President Pieter W. Botha refused to reopen the case despite widespread domestic and international appeals for clemency.

Benjamin Moloisi, 31, who is scheduled to be hanged Friday in Pretoria, would be the first black executed for a political crime since three guerrillas of the African National Congress went to the gallows in 1983.

The ANC, in a statement from its headquarters in Lusaka, Zambia, reiterated that Moloisi was not responsible for the policeman's death and said Moloisi's execution would be "judicial murder."