The third Jamestown (AG-166), a converted Liberty ship, was launched as J. Howland Gardner under Maritime Commission contract by New England Shipbuilding Corp., South Portland, Maine, 10 July 1945; and sponsored by Mrs. George W. Elkins of Newport, R.I. 

The liberty ship was completed 14 August and chartered under general agency agreement by Waterman Steamship Co., until 17 June 1946 when she went into the Maritime Reserve Fleet. She was chartered by U.S. Navigation Co., 3 February 1947, and by South Atlantic Steamship Lines, 20 October 1948. 

J. Howland Gardner returned to Maritime Reserve Fleet at Beaumont, Tex. She was acquired there by the Navy 10 August 1962: renamed Jamestown and designated AG-166 on 6 March 1963; and commissioned 13 December at Norfolk Navy Yard, Comdr. Alan J. Kaplan in command. 

The research ship was assigned to Service Squadron 8, Service Force, Atlantic Fleet, with Norfolk her home port. Her mission is "to conduct technical research operations in support of U.S. Navy electronic research projects, which include electromagnetic propagation studies and advanced communications systems such as satellite communications."

 After fitting out at Norfolk, Jamestown departed 20 January 1964 for Guantanamo Bay, Cuba, where she underwent 2 weeks of intensive shakedown training. She was there when Cuban Fidel Castro shut off all fresh water to that base, and stood by ready to evacuate American families. Upon completion of shakedown training, she made brief visits to Kingston, Jamaica, and Key West before returning to Norfolk 27 February.

 Jamestown was redesignated AGTR-3 on 1 April and 8 days later departed on her first deployment visiting Gibraltar; Valletta, Malta; Aden; Capetown; and Freetown, Sierra Leone, before returning Norfolk 17 August. The ensuing weeks were devoted to preparation for deployment off the African coast. She stood out of Norfolk 2 November for Dakar, Senegal, Capetown, South Africa, and further service acquiring new knowledge in the vital field of electronic communications. Jamestown then returned to Norfolk 6 February 1965. After operation in the Caribbean in the spring, she transited the Panama Canal for a cruise along the Pacific Coast of South America reaching Valparaiso, Chile, 9 June. She celebrated the Fourth of July at Callao. Peru, and then transited the Panama Canal, returning to Norfolk 23 July.

 Exactly 3 months later Jamestown got underway for the Far East and reached Subic Bay in the Philippines 29 December. She operated in the South China Sea gathering valuable information for the Navy's ships fighting to protect the independence of South Vietnam while adding to the long Navy tradition of serving the field of scientific research. She continued operating in the Far East, often operating in the Vietnam war zone, through mid-1967.