Dr. Christopher C. Fordham III, head of the University of North Carolina Medical School at Chapel Hill, is scheduled to become assistant secretary of health, education and welfare for health.

HEW Secretary Joseph A. Califano Jr. has chosen Fordham after a search in which three prominent doctors turned down the job.

They were David Hamburg, head of the Institute of Medicine at the National Academy of Sciences here, Charles Sandler, director of Massachusetts General Hospital, and William Roy, former Democratic representative from Kansas.

All gave personal reasons for declining, but two said privately that they were concerned because Califano had already made so many health decisions himself that he had reduced the job scope.

Califano has dismissed the previous health chief, Dr. Theodore Cooper, and several subordinate agency heads. He has announced that he was keeping Dr. Donald Fredrickson as head of the National Institutes of Health, asked Dr. David Sencer, head of the Center for Disease Control, to resign, and named Dr. Donald Kennedy head of the Food and Drug Administration. He has thus made many of the new decisions a new assistant secretary might ordinarily make.

He also announced that he is putting Medicare and Medicaid under a unified administration along with medical quality-control functions now under the assistant secretary a strong policy voice in all health decisions.

But some persons in the health field think one problem in American health care has been the severely divided responsibility inside HEW, with no one in charge of health as a whole except for an often overburdened secretary.

"I withdrew my name for family reasons," Roy, now head of obstetrics and gynecology at St. Francis Hospital in Topeka, said Tuesday. "But I hope all health is eventually put under one head."

Fordham, 50, is a Harvard Medical School graduate who served a 1951-52 internship at Georgetown University Hospital. He joined the University of North Carolina medical faculty in 1954, served as medical vice president of the Medical College of Georgia from 1969 to 1971, then returned to North Carolina as medical dean.