The Fairfax County School Board, expressing concern about overcrowded classrooms in the fast-growing western sections of the county, voted last night to urge that the timetable for the construction of three elementary schools there be speeded up by several months each.

The schools in question are Salisbury, scheduled to open in January 1987, which the board hopes could be opened the previous September; Sully, scheduled to open September 1987, which the board hopes could open the previous January, and Union Mill, which the board hopes could open in September 1986 instead of the following January as scheduled. Salisbury is in the Centreville area, Sully is in the Franklin Farms section and Union Mill is in Centreville.

Kohann H. Whitney, the school board member who sponsored the motion urging the speedier construction, said construction of the three elementary schools would eliminate the need for 15-to-20 temporary classroom buildings at other elementary schools.

The motion she sponsored requires school staff to study sending a letter to the Board of Supervisors asking that school construction be given top priority and that school staff meet with county building officials to try to expedite construction. Her motion also urged study of shorter-than-usual bidding timetables for school construction and the possibility that classrooms could be constructed and schools opened before auxiliary areas such as the gymnasium and cafeteria were ready.

School officials said after the meeting that contracts for the three schools already have been awarded, with set completion dates. Nevertheless, they said that they would study the proposal.