Three buildings are ready for occupancy at Avenel Business Park, a 12.6-acre office complex in Gaithersburg under development by Franklin Property Co., a division of B. F. Saul Co. The one-story, brick structures totaling 120,000 square feet were designed by Andrew V. Donnally and Associates.

The 30,000- 34,000- and 56,000-square-foot buildings at 201, 205 and 207 Perry Hwy. are suited to both research and development and high-technology tenants, said project manager Dane Adcock. Four additional single-story structures totaling 136,000 square feet are scheduled for construction this fall, he said.

Other recent developments in commercial real estate in the Washington area:

* Airline Tariff Publishing Co. (ATPCO), has leased 9,000 square feet in Gateway 1 Building at Dulles Airport in Chantilly. The lease brings the firm's total space at the airport to 43,000 square feet. ATPCO publishes lists of passenger and cargo fares and provides related services for the airline industry.

* Professional Records Storage, Inc., has leased 10,000 square feet at 35 K St. NE. The firm stores inactive office records and files.

* United Service Organizations, Inc., (USO) has bought a 55,000-square-foot office building at 601 Indiana Ave. NW. Plans call for renovation of the 10-story structure, which will become the USO's headquarters.

* A 40,280-square-foot building at 8989 Hermann Dr. in Columbia is under development by the Hermann Drive Partnership. The shell of the one-story structure in Sieling Industrial Center has been completed; interior spaces will be finished to suit the needs of individual research and development tenants.

* The Moore Co. has begun construction of a 12-story office and retail building at 1100 Wayne Ave. in Silver Spring. The structure, overlooking the Silver Spring Metro station, will have seven floors of office space, two for retail use and three levels of above-grade parking. Ward and Hall were the architects for the project, which is scheduled to be completed early in 1983.

* Edw. R. Carr and Associates will begin construction in July of a six-story office building at 5904 Richmond Hwy. in Alexandria, at the juncture of Rte. 1 and the Capital Beltway. The 75,000-square-foot structure, called One Beltway Center, was designed by RWK International.