NEW YORK — NBC erected a billboard on Broadway touting that Jay Leno is No. 1 in the latenight television ratings. In response, CBS this week put up its own billboard, with a photo of a grinning David Letterman, that boasts his “Late Show” is No. 3.Both billboards are visible from the Ed Sullivan Theater, where Letterman’s show is taped. “We put it up for the same reason we do everything else. It made us laugh,” said Rob Burnett, executive producer of Letterman’s show. For those keeping track, Ted Koppel and “Nightline” are No. 2.