An estimated total of 28,000 black workers went on strike in six South African gold and coal mines today and face the threat of dismissal by three major mining companies.

Union leaders warned that if the threatened dismissals took place, the strike might be extended to 22 other mines Tuesday and sympathy strikes by allied black unions could be expected.

Although the strike was proving less effective than black union leaders had hoped, its explosive potential caused gold-share prices to fall sharply on the Johannesburg Stock Exchange today.

The rand currency's exchange rate, on the other hand, recovered slightly following the announcement Sunday night of a freeze on payment of principal on the foreign debt and exchange controls to after a 35 percent crash last week.

Meanwhile, police reported that at least three more persons died today as violence flared in several centers, particularly Cape Town, where the police clashed with mixed-race and Asian schoolchildren.

Spokesmen for the three mining companies involved in the strike -- Gold Fields, Gencor and Anglo Vaal -- confirmed today that miners were notified Friday that any who joined the strike would be "in breach of their contracts" and subject to dismissal, but they denied that this amounted to an ultimatum, as the union claims.

The notices did not specify when the dismissals would begin, but National Union of Mineworkers officials said tonight that some already had taken place and that others were expected Tuesday.

The union's general secretary, Cyril Ramaphosa, said in an interview tonight that if mass dismissals took place, the NUM might carry out its threat to extend the strike to 22 other mines and call for support from a dozen allied black unions.

That could lead to a nearly general strike by black workers amid South Africa's violence, which has resulted in nearly 700 deaths during the past year.

The miners' strike appeared to be faltering after only 24 hours, but the mining companies, emulating the government authorities in many similar situations over the past year, seemed intent on trying to crush it, which could provoke a much bigger conflict.

The union knew it was in a weak position when it called the strike Sunday. Its main strength is among the workers in the other 22 mines, but it negotiated a wage increase a week ago with the companies that control those.

The other three companies, where the union is weaker, refused to match the wage offer. This led to the strike, forcing the relatively new black union into a test of strength at the mines where it is weakest. Before the strike, Ramaphosa warned that if strike-breaking methods were used, he would strike the other 22 as well.

The union expected 60,000 miners to strike at seven mines where it had declared a dispute with management, but it admitted tonight that only 28,000 had failed to go into the mines the first full day of the strike.

An unexpected feature of the strike is that mine workers have struck at some mines where no official dispute has been declared, while workers have failed to strike at others where the union declared a dispute and expected a greater response.

Ramaphosa said that this was because of intimidation by management at mines where the strike was scheduled and that the action of mine workers at the other mines shows there is strong basic support for the union's cause.

The union claimed in a statement tonight that intimidation by company security guards had limited the number of strikers and that the police had been called to several of the mines, where they arrested strikers. The mine companies deny that they are forcing would-be strikers to go underground, countercharging that the union is intimidating miners who are eager to work.

Independent verification of these conflicting claims has not been possible because the mining companies have closed their property to reporters.

In developments in South Africa's financial crisis, the Johannnesburg foreign exchange market responded to the government's creation Sunday of a two-tiered system for the rand as a means of discouraging the withdrawal of foreign investment. The market opened the bidding for the currency that is to be used for the domestic economy, the "internal" rand, at 41 U.S. cents; the price at close was 45 U.S. cents.

This was an almost 10-cent improvement on the all-time low the rand reached when the government suspended the market Aug. 27 following a run for repayment of South Africa's $16.5 billion short-term loans. The run occurred as foreign banks became nervous about racial unrest and the government's failure to start dismantling the apartheid system of white-minority rule.

This was still 10 cents lower than market analysts were predicting the rand would be after the freeze announcement, but economists pointed out that South Africa's central bank was now in a position to control the currency's exchange rate, and, as one put it, "If that is where it is, then that is where they want it to be."

Meanwhile, the new "external" rand was quoted on foreign exchange markets today at 36 U.S. cents. This meant that foreign investors could buy into the South African market at a 20 percent discount and still repatriate their dividend earnings in the higher rated internal rands.

The discount revealed how the new currency arrangement will act as a brake on disinvestment, although, as economists pointed out, the exchange controls mean that no new investment capital can come into the country, just as none can leave. A new foreign investor can only buy "internal" rands created by another foreign investor's withdrawal, leaving a static situation overall.

After its initial shock at the announcement of the moratorium on debt repayments and the stringent exchange controls, business here seems to have accepted that the controls were inevitable in the face of foreign banks' demands for prompt debt repayments.

Violence continued as South Africans continued to focus on their financial crisis. Police reported that the body of a black policeman who had been burned to death was found in a township south of Johannesburg and that a black man died in unrest near Port Elizabeth.

A teen-age girl was reportedly shot dead in a clash with police in the mixed-race township of Elsies River, outside Cape Town, where 29 blacks died violently last week.

Street barricades with burning tires were set up in several mixed-race townships outside Cape Town, and rioters stoned cars and set fire to delivery vehicles.

In Pietermaritzburg, Judge John Milne upheld several defense objections to an indictment in which 16 leaders of the activist United Democratic Front were charged with treason and terrorism. Milne ruled that the main charges should stand, but ordered the state to give more details on secondary charges. He postponed the trial until Sept. 18.

Four whites who were attacked in their car by funeral mourners in East London Saturday had first been involved in an accident in which 11 of the mourners were injured, according to news agency reports today.

The agency reports quoted some of the injured blacks as saying that the car crashed into a group of them as they walked home from the funeral rally and the white occupants were attacked after this. Two of the whites were killed and the other two injured.