THREE OF THE 25 largest real estate brokerages in the nation are based in the Washington area, and a fourth is in Baltimore, according to the Questor Strategic Real Estate newsletter.

The largest local concern is Long & Foster Realtors, which ranked sixth nationally with gross revenue of $65 million for 1984. According to the newsletter, published by Stephen Roulac & Co. of San Francisco, Long & Foster rang up 23,962 transactions last year -- a volume of $2.46 billion -- and employed 2,800 people in 64 offices.

Long & Foster also ranked sixth last year.

Shannon & Luchs Co., which had ranked eighth in last year's survey, fell to 11th this time around. The firm's gross revenue for 1984 was $46.8 million, down from $55.2 million in 1983, according to the newsletter. Shannon & Luchs tallied 12,000 transactions last year for a volume of $1.75 billion, and had 1,950 people in 45 offices, the survey said.

Mount Vernon Realty Inc. was 25th on the list with revenue of $20.5 million. It recorded 7,500 transactions, and had 41 offices and 1,800 agents. The volume of its residential business totaled $850 million.

The Baltimore firm of O'Conor, Piper & Flynn Realtors ranked 14th nationally with revenue of $39.6 million. This firm resulted from the recent merger of five smaller ones, and is termed by the newsletter the dominant company in the Baltimore market.

The newsletter noted that the 10 largest firms had revenue totaling $1.8 billion, 6 percent of the industry total.

The rankings are for all services, including commercial sales and leasing.

At the top of the list was Coldwell Banker Real Estate Services, with gross revenue of $825.7 million. Following were Merrill Lynch Realty, $323 million; Grubb & Ellis Co. of San Francisco, $215 million; Goldman Sachs & Co. of New York, $140 million (estimated); and Cushman & Wakefield Inc. of New York, $111 million (estimated).

When residential services alone are considered, Coldwell Banker and Merrill Lynch continue to rank No. 1 and No. 2, but Long & Foster climbs to third, Shannon & Luchs to seventh, O'Conor, Piper & Flynn to eighth and Mount Vernon to 14th. AMERICANS BORROWED $202.2 billion -- more than ever -- to purchase homes in 1984, the Department of Housing and Urban Development says.

In 1983, the figure was $201.9 billion.

Mortgage payments in December, including prepayments and refinancing, were $37.4 billion, 6 percent below December 1983.

HUD said it got the figures from reports filed by 1,500 lending institutions and agencies.

Savings and loans accounted for 43 percent of the $16 billion in loans originated in December. Mortgage companies made 26 percent while commercial banks had a 22 percent share of the market. Other lenders, including life insurance companies, made the rest. DEVELOPERS OF A PROPOSED 175-unit apartment house for the elderly in Montgomery County will voluntarily lower its density now that the county planning board has recommended that the project be denied, an attorney for the developers said this week.

Housing for the elderly is allowed by special permit in all residential zones of the county, but the board recommended last week that the permit be denied because the six-story apartment house would be too large for the mostly single-family neighborhood on 16th Street near the District line.

Stephen P. Elmendorf, attorney for Oxford Development Enterprises Inc., said the developer will lower the density of the building by an undisclosed amount and make some improvements to the sidewalks -- another concern voiced by the planning board.

"We have made some revisions based on the planning board's concerns, but I'm not at liberty to say what those will be," said Elmendorf.

The project is planned for a 4.2 acre tract that the developers have a contract to purchase, he said.

John Hoover, spokesman for the county planning board, said some neighbors were concerned about the density of the project and its effect on their neighborhood.

The matter now goes before the county board of appeals, which will have three months to make a final ruling on the project, said clerks to that board. PERSONNEL FILE . . . Frederic G. Gale and Edward J. Pinto have been elected senior vice presidents at the Federal National Mortgage Association; and Dennis G. Campbell and Richard M. Leegant, vice presidents . . . David S. Mercer has been elected president of the Community Associations Institute . . . The Building Owners and Managers Association International has named Lee Johnson legislative counsel, Doug Greenaway assistant manager of codes and regulations, and Henry Chamberlain director of public relations. All three positions are new . . . At Sequoia Building Corp., Mark W. Morgan has become chief operating officer, and John T. Conrad has been named director of construction . . . Gary T. Gill has become president and chief operating officer of MacKenzie & Associates . . . Quadel Consulting Corp. has named Amy Jones vice president, and Alice Barrett and Ruth O'Sullivan associates . . . Lessie W. Powell has joined the Enterprise Social Investment Corp. as director of finance and development . . . Kathleen D. Will has been named vice president of PaineWebber Mortgage Finance Inc. . . . KMS Group has named E. Kemper Sullivan and Dwight Taylor vice presidents . . . Wendy A. Jordan has been named publisher of Remodeling World magazine . . . Charles B. Carter has joined Henry A. Long Co. as marketing director for its Westfields Project . . . New assistant vice presidents at Smithy Braedon: R. J. Turner, Armond Spikell, Kevin S. Campbell and Robert F. Donhauser.