The United States, Britain and France jointly cast vetoes in the Security Council tonight to defeat four resolutions calling for sanctions against South Africa for its failure to grant independence to Namibia.

The votes represented the first open break between the Reagan administration and the Third World on an issue that the black African nations have chosen to single out as the prime focus of their concern.

The council's vote was delayed for four hours by last-ditch efforts to avert a showdown. But in the end an offer by the West to reaffirm the existing U.N. framework for Nbamibian independence was rejected by the Africans as insufficient.

China and the Soviet Bloc joined six Third World countries on each of the four resolutions, providing the necessary majority of nine votes. But the triple vetoes -- the first cast on this issue by the Western powers since 1976 -- sent all four resolutions down to defeat.

Spain and Ireland joined the Third World in voting for an embargo on oil supplies for South Africa, and there were 12 votes on the 15-nation council for a text reaffirming an existing arms embargo.

Both the United States and France said in explaining their votes that they would remain bound by the arms embargo, adopted in 1977.

But all three Western nations argued that the imposition of sanctions would have hurt the negotiating process that, they maintained, could still bring independence peacefully to Namibia.

U.S. Ambassador James J. Kirkpatrick pledged an American commitment "on the highest levels" to the drive for a negotiated settlement.

She said that Western foreign ministers meeting in Rome next Tuesday, wil prepare "specific proposals that we hope to discuss with the parties" and appealed to both the black Africans and South Africa to cooperate in seeking a solution to the Namibian problem.

But the leader of the 50-nation African group, Uganda's Olara Otunnu, insisted that the vetoes "mean only one thing -- strengthening the hand of the occupying power."

He reflected the African frustration at the failure of a four-year Western effort to negotiate independence for the territory, which is also known as Southwest Africa, through elections supervised by the United Nations.

That effort reached an impasse in January, when a conference in Geneva failed to achieve agreement on a date for independence -- a failure blamed generally on South African unwilliness to hold elections that were expected to result in victory for SWAPO, the guerrilla group seeking independence for the territory.

Even before the voting, the Africans warned that they would proceed to a special emergency session of the General Assembly, which would have before it similar calls for sanctions.

The Africans backed away, however, from an earlier threat to proceed to the assembly immediately, and instead scheduled a group caucus for Friday to consider their options.

Most diplomats expected a delay in assembly action until after the Rome meeting.

South Africa hardened its position with the announcement this week, on the eve of South African parliamentary elections, that the Pretoria government would never turn over the territory to SWAPO.

In the course of the 10-day council debate that preceded tonight's voting, the United States, Britain and France emphasized their intention to pursue the Namibian issue with South Africa. But they argued that the existing framework for independence -- proposed by the West and endorsed by the council three years ago -- must be modified if the impasse is to be broken. t

Specifically, they called for some form of guarantee that minority rights would be respected after the elections are held.

But the Africans, suspicious of Reagan administration overtures to the Pretoria government, insisted on an assurance that the Western consulations would not prove to be an excuse for delay that would block the momentum they had generated by the U.N. debate.

The sanctions proposed by the Africans include a total embargo on trade, travel, diplomatic representation and commercial credits, and a specific proviso for an oil embargo. One of the resolutions also would establish a U.N. committee to oversee compliance with the sanctions.

The Africans maintain that the assembly is empowered to adopt trade embargoes once the council has been blocked by a veto, and the African nations would be bound to abide by any sanctions they voted through.

This fact may give pause to South Africa's closest neighbors, whose economies depend to a large extent on the present extensive trade between themselves and South Africa.