In terms of plans for hiring over the next three months, employers across the nation have as positive an outlook as at any time in the past two years, according to a private study released yesterday.

Locally, optimism is at a higher level than national averages - particularly in Northern Virginia and the District. Among the country's regions, employers in the South continued to express the greatest optimism about the outlook for new hires in the near future.

The study by Milwaukee-hased Manpower Inc., a large temporary employment firm, showed a "slightly greater" proportion of nationwide employers expect increased hiring levels in July. August and September that in the previous quarter or during the same period in 1977.

One of the most significant findings was increased hiring plans by manufacturers of durable and nondurable goods and in the construction finance, insurance and real estate industries.

In metropolitan Washington, 44 percent of employers expect to hire new workers in the next three months compared with 40 percent three months ago. More than 50 percent of employers now expect their work forces to remain unchanged, none expect [WORD ILLEGIBLE] and 1 percent said they are unsure about hiring plans in the D.C. [WORD ILLEGIBLE]

Nationally, 34 percent of employers expect to add to payrolls, 57 percent do not anticipate any changes, 6 percent see layoffs and 3 percent are unsure, Manpower said.

Three months ago and at this time a year earlier, 33 percent of national employers expected to add workers, 58 percent saw no changes, 6 percent forecast cutbacks and 3 percent were unsure.

As it has for the past 15 months, the South continued to lead all regions with proportionately more employers expecting to hire (38 percent). In the West, optimism declined but still was above the U.S. average with 37 percent of employment expecting to hire. Trailing the national outlook is the Midwest, unchanged in the past year, and the Northeast - which has trailed national averages for two full years.

Manpower surveys a sample of 6,000 large public and private employers in 10 industry groups and 210 metropolitan areas for its quarterly reports. Among other findings.

Projections for hiring new workers in D.C. are the strongest to date in 1978 with 47 percent of employers planning increased work forces and 53 percent seeing no changes, compared with 38 percent and 62 percent, respectively, three months ago. Most of the increased hiring will be in the private sector.

Suburban Virginia employers are the most optimistic, with 63 percent planning additions and 34 percent maintaining current levels. Three months ago, the Northern Virginia outlook was even at 47 percent expect to hire and 47 percent planning no changes. Strongest gains are expected in construction, services and wholesale-retail trade.

Prince George's County is a local weak spot, with only 20 percent of employers expecting new hires. 63 percent forecasting no changes and 17 percent uncertain. Three months ago, 33 percent expected to hire, 60 percent of employers polled saw no change and only 7 percent were not certain of the outlook.