The United States Information Agency has leased the third and final building at the Federal Center Plaza at 400 C St. SW. USIA, now at 1776 Pennsylvania Ave. NW, will move its headquarters to the 400,000-square-foot building, which is being leased for 10 years at an estimated rental value of over $60 million.

The $150 million Federal Center Plaza includes the 529-unit Capitol Holiday Inn, which opened in December, 1980, and a 300,000-square-foot office building, completed in 1981, which serves as headquarters of the Federal Emergency Management Agency and the Federal Labor Relations Authority. The plaza is built on land leased from the District of Columbia Redevelopment Land Agency.

Donohoe Construction Co. is the general contractor, and leasing and management is handled by John F. Donohoe & Sons. American Security Bank provided the construction financing for the entire complex, which is owned by 500 C Street Associates, of which Donohoe Construction Co. is the majority partner.

Recent developments in commercial leasing in the Washington metropolitan area:

* Equitable Bank National Association has opened a new branch at 6 Montgomery Village Ave., Gaithersburg. The aggregate rental value of the long-term lease is over $500,000.

* DBA Systems Inc. has leased about 19,000 square feet of space at 11781 Lee Jackson Memorial Hwy. in Fairfax. The aggregate rental value of the long-term lease is over $1.4 million.

* Mission Engineering Corp. has leased about 7,000 square feet of space at Capital Office Park in Greenbelt. The aggregate rental value of the long-term lease is over $1 million.

* Palmer National Bank, a newly chartered Washington bank, has opened its office at 1667 K St. NW. The aggregate rental value of the long-term lease is over $5.2 million.

* Hoffman-La Roche Inc. has relocated to 1050 Connecticut Ave. NW, from 1775 K St. NW. The aggregate rental value of the long-term lease is over $1.1 million.

* Riggs National Bank Trust Offices has leased over 4,200 square feet of space at 4801 Massachusetts Ave. NW. The aggregate rental value of the long-term lease is about $700,000.

* Dorsey & Whitney, a Minneapolis-based law firm, has leased over 3,500 square feet of space at 1800 M St. NW.

* Ralph D. Kaiser Co., a brokerage firm, has leased 1,800 square feet of space at 2201 Wisconsin Ave. NW.

* Salvation Army national affairs office has leased 1,500 square feet of space at 1025 Vermont Ave. NW.

* John H. Quinn Jr., a Washington law firm, has leased about 1,500 square feet of space at 1000 Connecticut Ave. NW.

* Davis, Graham & Stubbs, a Denver-based law firm, has leased 7,500 square feet of space at 1001 22nd St. NW. The aggregate rental value of the lease is over $1 million.

* Blake Construction Co. Inc. has begun construction on a new 12-story office building on the southeast corner of 14th Street and New York Avenue NW. The building will contain about 130,000 rentable square feet of space, including about 7,000 square feet of retail space on the first floor. Completion is scheduled for August, 1983.

* Peoples Drug Stores is opening two new drug stores this month at the Willston Shopping Center in Fairfax and at the Hillcrest Heights Shopping Center in Hillcrest Heights.