Two generals and an admiral who served under the late Zairian dictator Mobutu Sese Seko are under investigation for alleged involvement in a plot to overthrow the new Congolese government, the South African Foreign Ministry said today.

The allegations follow Saturday's arrest of the three for entering South Africa as illegal aliens, using false passports.

A search of the men's exile homes here reportedly yielded documents that indicated involvement in the plot to topple President Laurent Kabila's government in Congo, as the former Zaire has been known since Kabila pushed Mobutu out of power last May after an eight-month rebellion.

A day before Kabila's forces seized Kinshasa, the Congolese capital, Mobutu fled into exile in Morocco, where he died in August. Some of the top figures in the ex-dictator's defeated military fled to South Africa, where many of their wives and children already had been living.

Among those exiled were the three men now under arrest: Gen. Nzimbi Etienne, former chief of Mobutu's special presidential division; Gen. Baramoto Kpama, former chief of the former Zairian civil guard; and Grand Adm. Mavua Mudima of the former Zairian navy.

These men, and several others, have been the target of a high-level investigation begun here after Kabila's government requested South African help in tracking the whereabouts of Mobutu's followers -- specifically how they and their families had come into South Africa and what assets they might have brought with them.

The officials of the former Zaire who now live here are suspected of having gained their immense wealth through the corruption that characterized Mobutu's 32 years in power. Last summer, Kabila's government gave South Africa a list of nearly 40 officials and powerful figures from the Mobutu era that Kabila wanted investigated in South Africa, a government source here said.

"There are a lot of irregularities concerning the families and their settlement here," said Capt. Gerhard Swart of the South African Police Service's Alien Investigation unit, which is part of the task force that has been probing the matter here.

Flying out of the Rand airport near Johannesburg, Nzimbi, Baramoto and Mavua traveled to an undisclosed town in Congo late Friday for a stay that was supposed to have been only a few hours, Swart said, but bad weather delayed their departure and the men had to return to South Africa on Saturday.

A tipster informed police here of their arrival and also of the fact that the documents under which they traveled were not legitimate, Swart said.

The men, who had been allowed to settle here on extended holiday visas, traveled to Congo under expired "stateless passports" that had been issued to them by South Africa in August for the sole purpose of attending the Mobutu funeral in Morocco.

But Friday, the men illegally used laminated copies of those documents to make their trip to Congo. The immigration official at the Rand airport who allowed them to leave the country on the false passports now also is under arrest.

In addition, a fourth exile who had attempted to intervene on behalf of his three compatriots was taken into custody on charges of making false declarations to immigration officials about his own status in the country.

Speculation in diplomatic and intelligence circles both here and in Congo has centered for several months on the suspicion that Mobutu's former generals would avenge their humiliating defeat by Kabila's rebel alliance.

South Africa's Ministry of Foreign Affairs said today that its officials are "seriously concerned about the alleged involvement of four citizens of the former Zaire . . . in a plot to overthrow the present government of {Congo}."

Renewed investigation will follow, the Foreign Affairs Ministry's statement said.

Jean-Lambert Ibula, a counselor at the Congolese Embassy in Pretoria, said his country's minister of justice will arrive in South Africa on Tuesday to deal with the case.

Although Congo has called several times for the extradition of the high-ranking officers to face charges of corruption and embezzlement, no extradition treaty exists between South Africa and the Congo, Swart said.

Kabila, whose new regime has met with mounting popular discontent, has what observers in Congo say is a tenuous hold on power, especially within his fragile military alliance.

That alliance was backed by foreign powers, chiefly Rwanda and Angola, and includes a hodgepodge of domestic and foreign soldiers as well as large contingents of troops co-opted from Mobutu's old army. Seventeen days ago, military tensions exploded in a day of street fighting in Kinshasa between Kabila's soldiers. CAPTION: Congo President Laurent Kabila reportedly was target of coup plot.