The term 'HABS' is an acronym for Historic American Buildings Survey, which is a program of the National Park Service. The program documents important architectural, engineering, and cultural sites in the United States. The program was established in 1933 and has since documented more than 38, 000 historic sites. The three-letter solution to the crossword is 'LES', which stands for the Library of Congress' Prints and Photographs Division, which houses the HABS collection.