Unemployment among District of Columbia residents has dropped by 3 percent since last year, almost to the level before the recession of the mid-1970s, the D.C. Manpower Department reported last week.

Since each 1 percent change in the employment rate represents about 3,300 jobs, the figures indicate that at least 6,600 more Washingtonians were working during March than during the same month last year.

Despite the rosier overall employment picture, city officials noted a continued slow erosion in the total number of jobs in the city, and continued joblessness among youths, notably young blacks, who experienced a 40 percent unemployment rate last year.

Officials said a seasonal rise in unemployment can be expected this month as thousands of high school and college graduates enter the work force.

Reflecting the recent decline in joblessness, unemployment compensation payments in the District of Columbia have plummeted by one-third in the past four months to the lowest level since 1975.

The jobless rate for city residents in March was 7.7 percent, seasonally adjusted, which is only 0.1 percent higher than the average jobless rate for all of 1975, when the total economy began a downturn. In 1976, the jobless rate rose to 9.1 percent, and in 1977, to 9.7 percent.

The Manpower Department said the greatest increases in employment in the city were in construction and wholesale and retail trade, following a normal pattern in the months when the weather improves. As reported earlier, a growing number of Washingtonians are finding jobs in the surburbs.

Despite a commerical building boom in downtown Washington, construction employment in April was still slightly short of the 14,300 jobs a year ago - apparently a result mainly of reduced Metro construction, which has moved largely to the suburbs and is tapering off overall.

William H. Alldredge, Metro construction director said the subway work force peaked at more than 8,000 in 1976, and now is only half that size.

Just last week, the federal government - responding to a lawsuit brough by the Washington Construction Industry Task Force - agreed to restructure and more strictly enforce a plan to increase the participation of minority workers on federally-financed construction projects in the region.

Such action would have its greatest numerical impact on workers who live in the District of Columbia, which is about 75 percent black.

In April, the Manpower Department reported, there were 574,900 jobs in the District of Columbia, a decline of about 2,000 from the same month last year. These jobs were held by both Washingtonians and suburbanites.

The 574,900 figure reflected a slight improvement in employment by private companies and a drop of about 3,000 jobs in government, which has been the city's only significant growth industry in this decade.

In 1970, according to government figures compiled in a report issued last month by Brimmer & Co., an economic consultant firm, there were 318,000 jobs in the private sector in Washington. By 1977, that figure dropped to 303,000.

The only industries listed by Brimmer that did not show declines were apparel sales, restaurants and financial businesses, such as banks, insurance companies and real estate firms.

During the 1970-77 period, the growth in government employment more than took up the slack caused by the downturn in the private sector, rising from 250,000 to 275,000 in that period.

In those same years, the number of private industry jobs in the suburbs jumped from 413,000 to 559,000, while government jobs in the suburbs rose from 201,000 to 249,000. Brimmer reported that blacks got 100,000 of the new suburban jobs.

Thomas A. Wilkins, director of the Manpower Department, said the declining jobless rate in the District of Columbia makes a continued high rate of unemployment among youth aged 16 to 19.

Rufus Daniels, a departmental economist, said the most recent available figures - for last year - show joblessness in this group hovered around 40 percent. Wilkins said there has been a slight but statistically insignificant improvement in that figure.

Nationally, last year, the overall rate of youth unemployment was 15.3 percent, but among blacks it reached 34.8 percent in July.

The falling overall unemployment rate for District of Columbia residents is mirrored by a sharp decline in payments made under the unemployment insurance program.

Unlike the figures for the unemployment rate, which apply only to D.C. residents, the payments made by the Board go also to Maryland and Virginia suburbanites who lost or were laid off their jobs in the city.

Such payments reached a recession peak in December 1975 when in one week 30.451 persons got checks totaling $2.8 million.

Since then, there has been a gradual downtrend, even when the unemployment rate for D.C. residents was rising.

By March 1977 about 20,000 persons were getting weekly checks for $2.1 million.

For the week ended March 3 of this year, 16,877 persons got checks totaling $1.8 million. For the week ended June 2, that number plummeted 32 percent to 11,457 beneficiaries receiving $1.2 million.

The number of applicants making their initial claims for benefits also has been dropping sharply, from 4,583 in January to 2,903 in May.

Because of a cash drain during the recession, the city's unemployment fund is insolvent. It had to borrow $67 million from the U.S. Treasury to continue paying benefits. That money must now be paid back.