Soviet President Nikolai Podgorny pledged today that his government will permanently support the just struggle of the fighters of southern Africa." His pledge followed a three-hour meeting with key nationalist leaders from Rhodesia, South Africa and Namibia (Southwest Africa) at the Soviet Embasy.

The session included Joshua Nkome of Rhodesia's Patrotic Front, Oliver Tambo of South Africas' African National Congress, and Sam Nujoma of Namibia's Southwest African People's Organization. A joint communique added. "The liquidation of the last vestiges of colonialism and racism is one of the most important international tasks."

All three african movements have long-standing ties with the Soviet Union, which has provided support and arms for guerrilla campaigns in Namibia and Rhodesia.

None of the nationalists would comment on the substance of the discussions. The only indication that the talks included the movements' future military plans was the presence of Soviet Gen. Sergei Sokolov, first deputy minister of defense and well-known "distributor" of arms to liberation groups.

Podgorny's host, Zambian President Kenneth Kaunda, speaking at a dinner, gave the first indication that the frontline African states - Zambia, Tanzania, Mozambique, Angola and Botswana - may not be totally committed to the Soviet-backed Patriotic Front in Rhodesia.

Referring to the question of eventual majority rule in Rhodesia, which nationalist's call Zimbabwe, he said:

"We have no intention of interfering in the internal affairs of an independent Zimbabwe. We will not choose leaders or determine the form of government. That is the sacred prerogative of the people . . ."

The statement may have come as a surprise to the 120-member Soviet delegation. A goal of Podgorny's visit to Zambia. Tanzania and Mozambique reportedly is to win full support for the four-year-old guerrilla campaign largely controlled by the Patriotic Front.

Kaunda's speech may imply that Zambia is still open to a settlement that would include the moderate nationalists.

One of the moderates pointedly excluded from the session with Podgorny, the Rev. Ndabaningi Sithole, said at a press conference here that he would not support a call yesterday by Bishop Abel Muzowa for a referendum to choose a Rhodesia nationalist leader.

Nkomo's group and Rhodesian Prime Minister Ian Smith have also condemned the referendum proposal.

[Cuban President Fidel Castro canceled his visit to Zambia so as not to conflict with Podgorny's according to a Zambian spokesman quoted by United Press International.]