Federal agencies across the nation estimate they will have to fire an additional 3,366 employes between now and mid-March for economy reasons.

Metro Washington, with 350,000 of the government's million-plus white collar jobs, has taken a large share of job cutbacks to date. Personnel experts estimate that as many as half of the 3,366 new cuts in the works will be made in the Washington area, which has about 13 percent of the federal population.

More than 2,300 federal workers formally have been notified they are to be fired, and 1,108 of them work in Washington, or the Maryland or Virginia suburbs, according to the Office of Personnel Management. It is monitoring agency RIF actions and plans layoffs and furloughs of its own staff soon.

Head-counters say that 4,148 federal workers were fired between Oct. 1 and Dec. 31, including 440 here.

Most of the workers fired between October and December came from Health and Human Services. Nationwide, it RIFfed 2,996 during the period. Agriculture fired 298 people, Labor 401 employes, Action 205 (nearly all here) and Consumer Product Safety Commission 90 people, according to OPM data.

The number of federal workers facing RIFs could climb dramatically in the next few weeks. Some agencies are not reporting firing plans to the OPM. Others are unsure how many people they will have to fire, or furlough, to meet new, lower spending targets imposed by Congress. Many federal agencies still have not had their fiscal year 1982 budgets (the year began Oct. 1) approved by Congress and, although most expect to be cut, they do not know how deeply.