Beginning a major expansion in the Washington area, F.W. Woolworth Company's Woolco division opened three new stores yesterday, announced plans for two more this fall and said it is planning additional units here next year.

The three Woolco opened yesterday were in former Giant Department Store buildings that were vacated in January when Giant Food pulled out of the Discount store business.

Giant will continue to run food store adjacent to the new Woolco units, on Annapolis Road in Bowie, Rockville Pike in Rockville and Leesburg Pike at Tyson's Corner.

Woolco had seven stores in the Washington suburbs before the current expansion began, has 10 today and will be at a dozen by fall, when the company takes over two former Dart Homes locations, at Manaport Plaza in Manassas and Rolling Valley Mall in Burke.

The five stores will added about 16,000 square feet of new store space in the Washington market and should give Woolco at least an additional $40 million to $50 million in sales here, industry sources estimate.

Nationally, Woolco is opening 19 new stores this year which will bring its total 295 stores. With sales of $1.5 billion last year. Woolco discounter by Discount Store News, a trade publication.

F.W. Woolworth Co. sales are expected to top $3 billion this year, including Woolworth and Woolco stores in Canada and Great Britain, plus the Woolworth-owned Kinney. Shoe stores and Richmand Brothers clothing stores.