The Washington Gas Light Company, seeking to improve service and enhance its competitive position in the energy market, said yesterday it will assign 1,800 of its 2,800 employes to three new company divisions serving Maryland, Virginia and the District of Columbia.

WGL Chairman Donald J. Heim named a vice president and general manager for each of the new divisions. Thomas A. Duckenfield, 50, will head District of Columbia Gas; Edmund W. Smallwood, 53, will supervise Maryland Gas, and Patricia M. Woolsey, 38, will run Northern Virginia Gas.

Washington Gas has a total of 572,000 customer meters in the metropolitan area, with 238,000 in Maryland; 187,000 in Virginia, and 147,000 in the District.

The company said gas rates would not increase as a result of the restructuring. The decentralization plan, a spokesman said, has been under discussion for about a year. The spokesman said no further changes were planned.

WGL, the spokesman acknowledged, finds itself in heavy competition on two main fronts: from the electric companies, who have been aggressively pushing the heat pump as a lower cost method for home-heating, and from the oil industry, whose prices have been sliding. Lower oil prices can be attractive to large users of fuel, who have the capability to use either gas or oil.

The heat pump war has been played out recently in a series of radio commercials in which electric heat pump advocates touted the benefits of their device while the gas industry charged that heat pumps produced unsatisfactory results. WGL has about 67 percent of the residential heating market in this area, a spokesman said.

The WGL spokesman said no decisions had yet been made on new office space or other housekeeping arrangements for three new divisions. WGL currently has facilities in each of the three jurisdictions. Walk-in service for customers still will be available at 1100 H St. NW.

Two main groups of employes will move into WGL's three new divisons. The first group includes those who deal with governmental jurisdictions on rates, regulatory and community affairs. The second group includes people who have customer contact and workers who handle appliance, credit and customer service.

The 1,000 employes who will not become part of the three geographical divisions include those in centralized accounting, finance and information systems.

Duckenfield, who will head District Gas, joined WGL in June as assistant general counsel. Since 1980, he served as clerk of the court for the D.C. Superior Court. Smallwood, head of Maryland Gas, has served as vice president for rates and regulatory affairs at WGL since 1978. Woolsey, head of Northern Virginia Gas, joined WGL in 1971 and, most recently, has been director of marketing services.