Owners of Prince Georges Plaza, one of the earliest of metropolitan Washington's regional shopping centers, are planning a "grand closing" ceremony for early August.

By then, Arlen Shopping Centers Co. vice president Charles Leibler said yesterday, construction of a covered mall and modernization of nearly all 90 store fronts will be completed.

In addition to a multimillion-dollar refurbishing, on which construction already has started, Leibler revealed yesterday that a third department store will be constructed at Prince Georges Plaza on East West Highway in Hyattsville.

Leibler declined to identify the potential new tenant but other sources confirmed that the J.C. Penney Co has been conducting negotiations with Arlen on the location. A 160,000 square-foot, full-line department store is being planned adjacent to an existing G.C. Murphy location.

The Arlen official said he expects to receive permits for the construction by late this year. Construction would start in 1978 and the Penney store would be opened late in 1978 or early the next year. Several parking decks, providing additional space for 400 cars, also would be contructed under the developer's plan.

Prince Georges Plaza was built by Eastern Shopping Centers, Inc., an affiliate of the Grand Union Co., starting in 1957. The center opened in 1959 with the Hecht Co. and 40 merchants and annual sales of $27 million. A Woodward & Lothrop branch and other stores were added in the mid-1960s.

Mark Newby, manager of the shopping center, said yesterday that annual sales of the plaza now exceed $60 million and Leibler said "conservative" projections are that by enclosing the mall, an additional $17 million in sales will be added annually.

A mall heated in winter and aid-conditioned in summer is expected to attract new customers as well as some business that may have been lost several years ago when Landover Mall was opened, Leibler said.

The entire 4,200-car parking lot will be resurinced and more energy-efficient lishning will be installed over the next six months in addition to the mall construction. Under the plans of architect Jerry Humphreys of Chattanooga, kinsky, live plantings and seating will be provided on several levels in the new mall, to be covered by sky lights.

Leibler said more than 400 construction jobs will be created by the mall enclosure. The expected increase in sales volume should add more retail jobs and the new department store is expected to employ about 450 persons, he added.

John Hall, manager of Wilson's menswear at the plaza and president of the merchants association, said all stores were planning to remodel their fronts and that some planned major interior renovations. Woodies already has announced plans to modernize all of its stores, including the one at Prince Georges Plaza.

"There is no question that we had to make a decision to modernize or sell," said Leibler of Arlen, which purchased the center about five years ago and planned the renovation earlier - a program blocked by a lack of adequate financing, he said.

The 856,000 square-foot center is located on 60 acres of the old Christian Heurich estate. A subway station, to be located on one side of the center, is scheduled to open in 1982 or 1983.