Three leading black national from Rhodesia, South Africa and Namibia (Southwest Africa) left here for Angola today, reportedly for a meeting with Cuban leader Fidel Castro. Their white "counterparts"-South African Prime Minister John Vorster and Rhodesian Prime Minister Ian Smith-held their own summit in Cape Town.

The three African leaders who flew from Lusaka today, after a meeting with Soviet President Nikolai Podgorny here yesterday, are Joshua Nkomo of Rhodesia's African National Council, Sam Nujomo of Namibia's Southwest African Peoples Organization, and Oliver Tambo of South Africa's African National Congress.

At an airport press conference, they announced plans to hold talks with Angolan President Agostinho Neto. But the presence of several Cuban officials who accompanied them, appeared to confirm widespread reports of a meeting with the Cuban president, now on a state visit there.

In Cape Town, the prime ministers of the two remaining white-ruled countries on the continent talked for 90 minutes, mainly about Rhodesia's efforts to establish majority rule without international backing and the rapidly escalating 4-year-old guerrilla war.

It was their second meeting this year about Rhodesia's 11-year-old constitutional crisis. The first was on Feb. 9 after the official collapse of the Angola-American-sponsored Geneva peace talks.

The flurry of activity among both blacks and whites vying for power in the three troubled southern African countries comes at a critical time.

The campaigns of Soviet-supplied guerrillas in Rhodesia and Namibia have escalated in recent months, as Western efforts at finding peaceful and internationally acceptable means of establishing majority rule in both countries have bogged down.

In South Africa, the Vorster government has not responded to any of the major demands put forward by militant youths during six months of sputtering riots in black townships last year that killed more that 500 Africans.

Both meetings are, in part, responses to the simultaneous tours of two Communist leaders - Podgorny and Castro - in southern Africa.

The three African nationalist have taken the opportunity of the visits to brief their supporters-and the case of the Soviet Union their supplier - about their plans.

Yesterday Nkomo, Nujomo and Tambo spent three hours with Podgorny. If the Castro meeting comes off, their trip to Angola could be even more interesting.

Cuba's commitment of more than 15,000 troops to the Marxist faction during Angola's three-way civil war in 1975-76 has led to speculation about Cuban involvement in Rhodesia's and Namibia's guerrilla campaigns - either with technicians and advisers, or troops.

The Cape Town meeting is also at least partially linked to the Podgorny - Castro tours. The South Africans and Rhodesians have become convinced that the simultaneous visits mean that there will soon be a major new Communist-backed challenge to their governments.

The "Communist threat" in southern Africa is an issue that could lead to greater South African support for Rhodesia. Talks on the implications of a more conspicuous Communist bloc profile on the continent would almost have been unavoidable at today's session, since it is one of the important issues*that binds the two governments together.