The city of Cambridge and the Massachusetts Bay Transit Authority are continuing efforts to extend the MBTA Red Line beyond Harvard Square to the proposed Alewife Brook Station near Fresh Pond.

Three possible station locations are presently under consideration as possible starting points for extension of the line:

* The existing station location at the kiosk area;

* North of Harvard Square on Mass Ave in the information booth-Flagstaff park area across from Church Street; and,

* Brattle Square.

Sverdrup & Parcel, consulting engineers for the MBTA, are conducting a thirty-eight week study to determine the final route.

Criteria for selecting the best extension route and station location include the effect of construction on Cambridge residents and businessmen, cost, engineering feasibility and construction time.

Robert A. Bowyer, director of the Cambridge Department of Planning and Development, estimated yesterday that the final station location and line extension will take eighteen months to design and about three years to build. Bowyer predicted that the project will be completed by 1980.

The northwest extension of the Red Line will improve traffic, parking and access problems of the Harvard Square area, the Department of Planning and Development said in a report issued earlier this year. The impact of these changes should reduce the pedestrian-vehicular conflict in Harvard Square as well as noise and air pollution in Cambridge, the report said