The retired board chairman of Lockheed Aircraft Corp. was shot to death along with his wife and housekeeper by intruders who broke into their Main Line mansion, police said yesterday.

The victims were identified as Courtlandt S. Gross, 77; his wife, Alexandra, 72, and their live-in housekeeper, Catherine Vander Veur, 69.

A roofing contractor who arrived at the four-acre estate yesterday morning discovered one of the bodies, said Lt. John Sheehan, commanding officer of the Lower Merion Township detectives.

"He noticed that there was a body on the floor near the back door of the house," Sheehan said. When police arrived, they found three bodies.

Salvatore Frustaci, deputy superintendent of the police department, said, "There was evidence of forced entry to the home. There's no question we are dealing with a triple homicide. As to motive, that's sketchy right now."

The three "died as a result of gunshot wounds," Frustaci said.

The two-story Gross estate, valued at $500,000, is on Philadelphia's suburban Main Line and is located behind the home of Dr. Richard A. Davis, brother of First Lady Nancy Reagan.

Gross, who retired from Lockheed in 1967, became active in the company in 1932 when his brother, Robert Gross, bought and reorganized it. The brothers helped turn Lockheed Corp. into one of the pioneering giants of the aerospace industry.

Courtlandt Gross directed negotiations with the British government that led to purchase of 250 Lockheed-built Hudson bombers on the eve of World War II. The $25 million deal was largest contract awarded to a U.S. aircraft manufacturer to that time and marked the beginning of true mass production of airplanes. CAPTION: Picture, The Philadelphia Main Line estate of Courtlandt St. Gross, the retired Lockheed Aircraft Corp. board chairman, where the triple homicide took place, is valued at $500,000. AP