Three African governments that have been the leading advocates of a "dialogue" with South Africa's ruling white minority have concluded that their peaceful approach is not producing significant change in South Africa's apartheid policies.

Liberia, Senegal and the Ivory Coast now see direct and strong economic pressure from the United States as the key to moving the white government away from political repression of the black majority, leaders from those countries said in interviews last week.

"We have been disappointed that more has not been done to match the words with actions," said Liberia's president, William Tolbert, who welcomed South African Prime Minister John Vorster to Monrovia in 1975. This year his government is contributing $600,000 to a fund that supports African guerrilla movements fighting South Africa.

The change in mood in West Africa follows extensive racial violence in South Africa in 1976, a crackdown on multiracial groups in 1977 and a continuing increase in polarization between the country's 4.5 million whites and 18 million blacks.

One of the principal hopes for dialogue, which was launched in 1971 by the Ivory Coast's president, Felix Houphouet-Boigny, was that it would spur increased racial tolerance and positive interracial contacts.

"We hoped the South African whites would understand the need for change," Minister of State Auguste Denise said in Abidjan, the Ivory Coast. "But it seems that they did not understand. Vorster's government has told us they cannot make changes without facing a revolution," added Denise, who was acting head of state last week during Houphouet-Boigny's absence.

The new activism by Washington in southern Africa on the side of majority rule has produced another change in attitudes in this part of the continent. Moderate leaders say the Carter administration's southern Africa policy has made it easier for them to endorse American stands on other issues in African forums where such moves previously met with suspicion and criticism.

Yet they also stress the need for the Carter administration to move more convincingly on South Africa.

"In general, we approve the approach on Namibia and the Anglo-American plan for Rhodesia," said Senegal's prime minister, Abdou Diouf, in Dakar. "But more has to be done on South Africa. The United States has to adopt a plan for South Africa with deadlines for action. It has to be studied very carefully, but they have to study economic pressures. All other methods have failed."

The three leaders firmly connected their calls for more activism in South Africa with new appeals to the Carter administration to be more active also in confronting what they see as growing threats from Soviet and Cuban expansionism in Africa. They praised the airlift support Washington gave this month for Zaire when it was attacked by a rebel force.

"African moderate states were beginning to lose confidence when we saw the Carter administration doing nothing," Diouf said."This action will help restore credibility. But more has to be done to help the moderates defend themselves."

Houphouet-Boigny stunned other African leaders in 1971 by proposing to visit South Africa to start a dialogue that would discuss apartheid and giving blacks political rights. He offered this as an alternative to the routine condemnations of South Africa that came from pan-African organizations and then embryonic guerrilla efforts.

The dialogue campaign was launched as the Nixon administration sought increased "communication" with an international acceptance for the white government. France, the former colonial power in the Ivory Coast and Senegal, was also on good terms with Pretoria.

Despite visits by Vorster to Liberia and by senior South African officials to the Ivory Coast, African, American and French policies toward Vorster's government have hardened significantly. The South African invasion of Angola, the harsh repression of internal dissent and the arrival of the Carter administration were factors in the change, which has left the African proponents of dialogue with no visible results to justify their initiative.

"We would never rule out completely the concept of dialogue," Liberia's President Tolbert said. "We are still talking to Mr. Vorster, with frankness . . . but our $600,000 donation [to the fund to support guerillas] shows that we have come to believe that armed struggle yields some benefits also."

In addition to calling on South Africa to help bring independent majority rule immediately to Namibia (Southwest Africa) and Zimbabwe (Rhodesia), Tolbert said Liberia was also calling on the South Africans "to reorientate their thinking" and grant equality to their black citizens.

"The involvement of Russia in that region would be disastrous," he added. "People's attitudes must be affected," and American economic pressure is needed to bring those changes before the Soviets move into southern Africa, Tolbert said.

"An effort on the economic front seems to be the only way to get whites to press Vorster for change," agreed Denise of the Ivory Coast. "America can lead the way to a reconciliation." Houphouet-Boigny has not yet gone this far in his public statements on new pressures to augment dialogue.

The United States has opposed unilateral or United Nations economic sanctions against South Africa despite the growing calls for such action from African moderates and radicals alike.