The Gentlemen Songsters, a barbershop harmony singing group based in Lowell, will compete on Season 3 of Sing That Thing!, WGBH s competition show for New England s best singing groups. They ll perform alongside The Thursdays from Chelmsford High School and Noteworthy from Emerson College. The episode airs Friday, April 21, at 8:30 p.m., on WGBH 2.