Three Republican congressmen acted yesterday to block the Department of Energy's proposed takeover of the Forrestal Building at L'Enfant Plaza, charging that lavish and unnecessary renovations - such as $32,013 for three restrooms - are being prepared for Secretary James Schlesinger and his top aides.

"We are concerned about possible gold-plating and overruns," said Rep. William F. Walsh (R.-N.Y.), who accused the Carter administration of "blindly pushing ahead to install Mr. Schlesinger in a prestigious showcase, regardless of the cost."

Frank Pagnotta, director of Schlesinger's office, said, "there is nothing lavish about our plans." He said that the secretary's suite would include a lavatory, toilet and shower, and the deputy secretary and under-secretary each would have a bathroom "not as big as a closet."

"They are entitled to more," Pagnotta said, "but we didn't give it to them." He added that the various assistant secretaries also are entitled to private bathrooms, under General Services Administration regulations, but will not get them.

Walsh, ranking minority member of the House subcommittee on public buildings and grounds; Rep. William H. Harsha (R-Ohio), ranking GOP and Transportation Committee, and Rep. Barry M. Goldwater Jr. (R-Calif.), introduced a bill yesterday that would prohibit further alteration of the Forrestal Building until the House approves a prospectus submitted by GSA.

Rep. Norman Y. Mineta (D-Calif.), the subcommittee chairman, responded by scheduling a hearing tomorrow on the GSA plans.

Rep. Harold T. Johnson (D-Calif.), chairman of the full committee, said the subcommittee would study the prospectus "in great detail; I don't know a great deal about it."

Walsh said he is convinced that "a totally separate, new building, large enough to house all the department's Washington - area people, would in the long run be cheaper . . . and more efficient" than renovating a building that was constructed for the Defense Department as a Little Pentagon. The building was opened in 1970.

Walsh noted that Schlesinger has testified that the Forrestal would accomodate only about 5,000 of the new agency's current 8,000 area employes, "not to mention any increases," Walsh predicted that "we are going to be faced with an endless series of piecemeal projects whose costs will run off the scale."

Pagnotta countered that a new building "would cost $600 million and take seven years to get - we can't wait that long."

Harsha said $17 million has been appropriated to reimburse GSA for the renovation, and $682,502 of that had been spent, even though the Public Works Committee has not authorized the expenditures.

To make room for the energy workers, about 2,800 defense employes will be "parceled out all over Washington," Goldwater said. And, he added, the government will have to lease space for the other 2,200 DOD workers who will move out of Forrestal, at a cost of another $60 million.

Goldwater said he "reluctantly voted to . . . establish a new bureaucracy," believing it was necessary to have "a cohesive effort toward assembling independence." but in assembling the various energy-related offices in the building at 10th Street and Independence Avenue SW, the government "played havoc" with defense employes.

IN addition to finding new quarters for the group of 2,200 DOD workers, Goldwater said another 1,200 would be transferred to Buzzard Point; 1,200 to the old Energy Research and Development Administration headquarters on Massachusetts Avenue; 187 to Fort Detrick in Frederick; 73 to Bethesda, and three to Fort Lee, Va.

"Apparently some of the military personnel in the sensitive Air Force Office of Special Investigations will be moved to Albuquerque, N. M., BUT NO ONE KNOWS WHY THEY WILL BE SENT/,???MILES AWAY," Goldwater said.

A GSA spokesman said last night that the top floor of the Forrestal Building has been vacated by defense, but only a few energy employes have moved in so far. The complete switch will take a couple years.