In the wake of the recent $30 million-plus sale of 3,700 acres of undeveloped land in Reston to a Mobil Corp. subsidiary, three former top officials of the seller, Gulf Reston, Inc., have moved into similar positions with Mobil.

James W. Todd, who was president of Gulf Reston, a subsidiary of Gulf Oil Corp, that has been developing the 15-year-old new town in Fairfax County for more than 10 years, now heads Reston Land Corp., a new subsidiary of Mobil Land Corp. Francis Steinbauer and Lee Shur have moved with Todd into executive posts.

The Mobil subsidiary plans to develop new residential areas, commercial and light industrial facilities on the remaining undeveloped area of the 7,700-acre new town originally opened by Robert E. Simon in 1964.

Meanwhile, Gulf Reston now has William C. Cox, a career executive with Gulf Oil and its subsidiaries, as president and chief executive officer. Gulf Reston has set up its program to sell income properties in Reston that include 1,000 rental apartments, industrial buildings, a high-rise office building with 227,000 square feet of space, and three village center shopping areas.

The price of more than $30 million that Mobil paid for the undeveloped Reston acreage was released recently by Coldwell Banker, a national real estate firm based in Los Angeles (with branches in this area) that was agent in the transaction.

Reston, which now has approximately 25,000 residents in a variety of dwellings and is home to 25 national organizations and trade associations, is located in upper Fairfax County on the former Sunset Hills farm, several miles east of Dulles AIrport.