The ratio of AB:BC is 3:1, which means that for every 3 units of length that AB has, BC has 1 unit of length. Let the distance between A and B be 3x, and the distance between B and C be x. Then the ratio of AB:BC is 3x:x, which is equivalent to 3:1. Therefore, x=1. The distance between A and B is 3*1=3, so B is located at 6+3=9 on the number line.