Three of the highest-ranking blacks in the Reagan administration yesterday criticized U.S. blacks for focusing on South Africa while critical problems persist at home.

The three -- Clarence Thomas, chairman of the Equal Employment Opportunity Commission; Clarence M. Pendleton Jr., chairman of the U.S. Civil Rights Commission, and Steven J. Rhodes, assistant to the vice president for domestic policy -- said they oppose apartheid but gave unqualified support to President Reagan's policy of "constructive engagement" with South Africa.

Samuel R. Pierce Jr., secretary of Housing and Urban Development, in a statement also said he opposes apartheid but did not disclose whether he backs the administration's South Africa policy or a current wave of protests against it.

Thomas, Pendleton and Rhodes, among the few high-level black political appointees working for the administration, said they sense a lack of attention to ongoing budget and tax deliberations from black leaders distracted by mushrooming South Africa demonstrations.

They also stressed that past policy moves to deny U.S. aid or limit U.S. business in South Africa failed.

"What about the budget? What about taxes?" asked Rhodes. "These are of direct importance to black Americans. If you want to be in the mainstream of power, that is where you've got to be. Regardless of what we do in the United States, the South African government will determine what happens over there. Black leaders could have more impact in directing federal spending to people who depend on those programs."

Rhodes said the administration is considering asking "distinguished" black Republicans, such as William T. Coleman Jr., former transportation secretary, and Edward W. Brooke, the former senator, to publicly endorse its policy.

Reagan is set to meet today with South African Nobel Peace Prize winner Bishop Desmond M. Tutu. "We're doing good over there, but so far people haven't bought it," Rhodes said. "Tutu might help."

"All of us who have lived under segregation, a mild form of apartheid, are concerned," said Thomas. "But in terms of the immediate, in terms of priorities, I think we should focus more on what is happening here. If these were protests about the quality of education black kids in the United States receive or about the high crime in black neighborhoods . . . I would be right out front in that kind of march."

Pendleton said the marches have "pricked the conscience" of America. But he asked whether black protesters at the South African Embassy have weighed the U.S. interest in a stable South African government against their concern about apartheid.

"If a black is on the defense committee and rises to the top, would he vote to send troops into South Africa to defend American interests or have this pigment attachment come first?" Pendleton asked.

"If we had to send troops into Poland, would a Polish committee chairman from Chicago hesitate to protect American interests because of his heritage? I don't know how you deal with it. Are they going to be just blacks or go beyond that?"