Employers in metropolitan Washington are a bit more pessimistic about expanding their work force during the balance of 1978 than they were three months ago.

According to a national survey by Man Power Inc., a Milwaukee temporary help company, 41 percent of area employers except hiring increases, 4 percent expect declines, 54 percent forecast no changes and one percent is untrue.

Three months ago, 44 percent of the firms polled expected hiring increases and 51 percent projected no changes.

Although area employers are slightly more pessimistic, the Washington area index continued to exceed national averages. Across the nation, only 30 percent of employers said they expect new hires while 9 percent forecast cutbacks and 58 percent saw no changes. The most recent U.S. data also represent a decline in favorable hiring plans from three months ago.

For the first time in Manpower's studies, conducted by interviews with a sample of 6,000 large public and private employers, hiring expectations in the West trailed all other regions of the country - apparently reflecting plans for less, hiring in California after approval of Proposition 13 that slashed property taxes.

The South continues to lead all regions in every industry sector, with forecasts of substantially greater hiring expectations.

In the D.C. area, the brightest employment outlook was given by employers in Northern Virginia. Some 53 percent of employers there expect to expand their staffs while 47 percent see no changes.

The worst outlook is in Prince George's County, where only 21 percent see expansions, 65 expect no change and 7 percent predict cutbacks.

In between these area extremes are the District (42 percent expect expansion, 58 percent, no change) and Montgomery County (44 percent expect expansion, 44 percent see no change and 12 percent forecast cutbacks).

Real estate-finance-insurance, construction, manufacturing and wholesale-retail trade firms expect to add to their staffs in Northern Virginia.

Another survey published yesterday, by the New York executive search firm of Korn/Ferry International, showed that U.S. corporations hire 46 percent more executives in the third quarter than in the same period last year. The year-to-year gain was the largest in the past seven years.

Separately, the Virginia Employment Commission said that the states jobless rate in August was 5.2 percent, the same rate as during July. A spreading rail strike against the Norfolk & Western - ended temporarily over the weekend - prevented a normal seasonal decline in the state's jobless rate.