A major expansion of Tysons Corner Shopping Center will bring three department stores, including Saks Fifth Avenue and I. Magnin, to the Fairfax County mall, the busiest in the metropolitan area.

The three stores - the name of the third has not yet been disclosed - would add about 400,000 square feet to the 1.5-million-square-foot retail center, which has 140 outlets. The stores could open their doors in three to five years, based on normal development timetables.

In coming to Tysons, Saks and Magnin would be opening their first outlets in Northern Virginia. In the past, Saks has considered coming to the long-planned retail center near the Pentagon and Magnin considered locating at Fair Oaks, the new Taubman Company mall under construction at 1.66 and Rte. 50 in Fairfax.

Sources close to the Lerner Corp. which owns Tysons, said expansion of the existing mall would have no effect on plans by Lerner to construct a second mall - Tysons II - on 107 acres just across Dolley Madison Boulevard, which bisects the two properties.

"The second mall is going ahead," the sources said, adding that the protracted local-state-developer negotiations on major road improvements in the badly congested Tysons area are moving along and should be complete "early next year." Tysons II is scheduled to open in the early 1980s.

The 10-year-old Tysons I has 225,000 visitors daily, making it the busiest in the area and the fourth busiest in the cuntry. Its last major expansion came in 1976, when Bloomingdales opened after enlarging the long-vacant Lansburgh's, one of the three original department stores anchoring the mall (the other two are Hecht's and Woodward Lothrop, which also expanded.)

Saks and Magnin both of which cater to a clientele affluent enough to spend $45 for a paperweight or $4,500 for a fur coat - at present have only two outlets in the region, both in suburban Maryland. Neiman-Marcus, another affluent retailer with a D.C. store close to Saks, has expressed interest in a suburban Virginia location for a second area store.

The Saks store has long been a fixture at the Chevy Chase shopping district near the District line and Magnin is one of the anchors of the recently opened White Flint mall on Rockville Pike.

According to site plans submitted to Fairfax, all three stores, including the unnamed third one, would occupy a total of about 345,000 square feet.There would be three decks of parking above present spaces to accommodate additional traffic which the stores are expected to generate.

Currently, there are 6,500 parking spaces at the mall, and during busy periods - such as the present Christmas shopping season - all those spaces are occupied.

In the 1970s, Tysons Corner, through which about 50,000 cars and trucks pass daily, has become what Fairfax Supervisors Martha V. Pennino (D-Centerville) calls "the downtown of Fairfax County."

As recently as the mid-1960s, apples and other produce could be purchased from a farm stand where the shopping mall now stands. But as Fairfax has been subjected to rapid urbanization, so too did strategically located Tysons.

Besides the Lerner-owned mall and a smaller one nearby, Tysons is also the center for major commerical space - present, under construction or planned - amounting to more than 6 million square feet. There is also a new Ramada Inn, a Best Western under construction, an expansion planned for the present Holiday Inn and reportedly plans for the first Marriott in Fairfax.

Officials of Saks and I. Magrin confirmed that they are negotiating it with Lerner about locating stores at Tysons Corner, but said no contracts have been signed. A Saks spokesman said the company hopes to build a full-size store comparable to its one at Chevy Chase, somewhere in Northern Virginia.

Magnin also has indicated it plans a second store, but its move could be in doubt because the company's two officials were ousted last week in a major management shakeup.