Initial domestic air travel on Concorde supersonic jets could begin in October between Washington and Dallas-Ft. Worth, in the wake of a tentative decision yesterday by the Civil Aeronautics Board.

Three members of the regulatory agency, a majority, said yesterday they would approve a proposal by Braniff International to interchange with Air France and British Airways flights that now terminate at Dulles International Airport. The CAB staff was directed to draw up a formal order.

Braniff crews would operate the Concordes between Texas and Dulles, a first in the U.S. airlines industry, and Dallas-Ft. Worth would become the ninth airport in the world to receive Concorde service including Dulles and New York's Kennedy.

Between Texas and Dulles, Braniff plans to fly the Concordes at subsonic speeds. But at 95 percent of the speed of sound compared with 80 percent for conventional jets, the Braniff flying time would be reduced by 12 percent, or 22 minutes, making the flight time 2 hours and 8 minutes.

Total time for flights from Texas to London or Paris would be 7 hours, including the [WORD ILLEGIBLE] at Dulles.

Braniff plans to carry passengers between Washington and Dallas-Ft. Worth as well as those traveling between Texas and Europe. The fare between Dulles and Texas would be $177, representing a 10 percent surcharge on the current first-class fare of $161.

Braniff Chairman Harding Lawrence said yesterday that he hopes to start the service in October. The CAB still must vote formal approval and the Federal Aviation Administration is working on proposed U.S. certification of the aircraft - which an agency spokesman said should be completed by September.

The Dallas-based airline plans to operate Concorde arrivals and departues six days a week from Dulles. Braniff will lease the planes for domestic service from the French and British carriers. Meanwhile, in unrelated aviation developments.

The CAB gave tentative approval for 3 percent fare increases between Hawaii and the mainland, sought by Continental and Western airlines.

Colonia Airlines, a New Jersey firm that has proposed Washington-Newark-Boston shuttles as initial airline operations, will be granted a certificate to become a regular carrier, under a tentative CAB decision.