Allied Stores Corp., one of a handful of the nation's largest department store companies, somehow has avoided until now the country's most affluent retail market--metropolitan Washington.

But, by a coincidence of events that could not be foreseen three months ago, Allied is about to become a major factor in Washington business.

One week from today, Allied's Plymouth Shops division will open three stores in downtown Washington--at 1110 Vermont Ave. NW, 500 C St. SW and on 18th Street NW between K and L streets.

Plymouth caters to working women with a taste for fashion, and among the nation's 10 major metropolitan areas, Washington has the highest proportion of working women.

Allied expects good results from Plymouth, which has concentrated on the New York market until now. Company Chairman and Chief Executive Thomas Macioce said last week that he expects three more Plymouth stores to open in the D.C. market.

Separately, on Nov. 10, the day after Allied opens its Plymouth stores, the New York retail firm will take over a large, existing company based in Washington -- Garfinckel, Brooks Brothers, Miller & Rhoads, Inc.

The final stockholders meeting for Garfinckel, Brooks Brothers is scheduled that morning. Within a few minutes, the business of that session is expected to be completed, and the women's retail company started by Julius Garfinckel in 1905 will cease to exist as an independent operation.

If nothing else, when Allied votes the 92 percent of shares it owns already in favor of the final merger, one of the most cumbersome names in corporate history will be eliminated. Garfinckel, Brooks Brothers, Miller & Rhoads represent the names of three key retailers that have been combined over the years as the Washington company expanded into a nationwide group of 236 department and specialty stores in 36 states, with annual revenues of close to $500 million.

In an interview last week, Macioce indicated that the Garfinckel business would continue on the path of planned growth but that there would be changes in supervision:

* The department stores--Miller & Rhoads of Richmond, Harzfeld's of Kansas City and Miller of Tennessee--will be put under the direction of Garfinckel President Manuel Rosenberg, who will report here to Allied Executive Vice President John Cullen.

* A specialty store group, including the Garfinckel chain, Ann Taylor stores and Catherine's Stout Shoppes, will report to Garfinckel Chairman David Waters, who in turn will report to Macioce.

* Brooks Brothers, regarded as the best managed and most profitable men's apparel chain in the nation, will operate as a separate division based here under its president, Frank T. Reilly. Brooks has annual sales estimated at more than $120 million with a pretax profit margin of 18 percent--accounting for more than a quarter of Garfinckel's overall sales and the bulk of its profits.

Macioce said expansion plans of all the Garfinckel divisions would move forward--except for an estimated $100 million of new store spending to expand the 22-store Miller & Rhoads chain in Virginia and North Carolina.

Miller & Rhoads currently is losing money; estimated losses in 1980 exceeded $1 million on sales of $125 million. Macioce said Allied's first priority next week will be to beef up this department store division by modernizing existing stores.

On another topic, Macioce said his company's Bonwit Teller specialty division would no longer look at the Washington area for possible expansion. Allied acquired Bonwit in 1979 and Macioce said Garfinckel's "basically is the same type of store."

Overall, Allied had sales last year of $2.3 billion in 226 stores across the country, including Jordan Marsh in New England, Gertz of Long Island, Stern's in New Jersey and Joske's of Texas.