Why does Aaron Rodgers say 319?

Answer 1

It's "green" 18, not "3" 18. It's the start of his snap count. The ball is then snapped on a one or two count following. He conditions the defense to assume that the ball will be snapped on one or two. He then sometimes shouts an exaggerated hut-hut following green-18 but the ball is not snapped (called a "hard count") to try to draw the defense offside. The hard count only works if the quarterback has been consistent in his pattern up to that point.