The National Capital Planning Commission yesterday unanimously approved construction of three new federal office buildings in Washington's downtown Federal Triangle.

The decision clears the way for the federal government to move ahead with the massive $350 million project. Congress is expected to approve $3.5 million in planning funds in the 1983 budget, paving the way for a national design competition for the buildings. A federal building fund already contains sufficient money to construct the project.

Preliminary plans call for one building facing Pennsylvania Avenue, between the District Building and 13th Street NW, and a pair of 10-story office buildings with a 10-story glass gallery between them on the Great Plaza just behind the District Building. Parking lots now occupy both sites.

As many as 6,500 federal employes and another 1,500 private workers would occupy the new buildings, adding to the19,000 employes who already work within the triangle. Exactly which federal employes or agencies would move will not be determined until final designs for the building are complete, a General Services Administration spokesman said yesterday.

The planning commission urged GSA yesterday to slightly reduce the size of the twin office buildings and to reconsider the shape of the proposed semicircular building facing Pennsylvania Avenue. The Fine Arts Commission, which recently gave its general blessing to the project, also expressed some concern about the shape of the semicircular building. Both groups must approve the final design of the project.

In approving construction of the office buildings yesterday, the planning commission rejected the recommendation of the Joint Committee on Landmarks, which urged that the Great Plaza be developed as the "magnificent garden park" planned when the Federal Triangle was built in the 1920s and 1930s.

The Depression halted completion of the plaza, courtyards and small parks and portions of some of the seven federal buildings within the triangle. It also halted planned demolition of the only older buildings now remaining in the triangle--the 1890s romanesque Old Post Office building, restored and to be reopened this fall, and the 1904 beaux arts District Building.

GSA plans for the Federal Triangle include completing the bare-brick facades left on two buildings--the Internal Revenue Service and new Post Office buildings--opening courtyards and developing a landscaped "federal walk" through the triangle, to be open to the public from Sixth to 15th Street.

The GSA plan envisions a major expansion of commercial space, including restaurants and cafes, within the Federal Triangle to enliven the area. A total of 3,170 underground parking spaces will be provided.