Social welfare spending by public agencies, which grew dramatically in the three decades after World War II, has dropped for the third consecutive year as a percentage of the gross national product, according to a report published yesterday by the Department of Health and Human Services.

The report does not reflect the expected consequences of deep budget cuts in welfare, Social Security, child nutrition and other federally funded social programs being sought by President Reagan. It reflects only spending trends under way before Reagan came to office.

The report shows that growth of government burdens for the poor, the disabled and the elderly, which the Reagan administration has criticized as harmful to the economy, stopped before Reagan became president.

In 1950, the report shows, federal, state and local governments were spending $23.5 billion, or 8.9 percent of the GNP, on programs such as welfare, Social Security, unemployment insurance, public housing, education and public health programs.

Powered by the growth of Social Security and enactment of programs such as Medicare, this figure rose rapidly over the next generation and reached a high of 20.4 percent of the GNP in 1976, according to the report.

In 1977, the figure was 19.6 percent, and in 1978 it was 19.3 percent. In 1979, total spending for social welfare by government agencies at all levels was 428.4 billion, or 18.5 percent of the GNP.

Moreover, according to the report, it terms of constant 1979 dollars, the 428.4 billion figure was a decrease of $700 million from the 1978 figure.

Reagan and aides including Office of Management and Budget Director David A. Stockman have said frequently that the government's burden in social welfare has been growing too rapidly and should be pared.

Joseph Pechman, director of economic studies at the Brookings Institution, said one reason for the drop in social welfare outlays as a percentage of GNP is the government's failure to raise various welfare payments enough to keep pace with inflation.

"The country is not willing anymore to allocate an increasing percent of its wealth to social welfare programs. Even without Reagan, the figure was going down a bit," Pechman said.

"With Reagan's cuts, it will go down very substantially, especially as cuts are made in social programs to meet future administration targets."