What are the characteristics of numbers divisible by 3 and 9? Five examples

What are the characteristics of numbers divisible by 3 and 9? Five examples

The sum of the numbers is 9
9 (9=9) 9/3=3 9/9=1
18 (1+8=9) 18/3=6 18/9=2
108 (1+0+8=9) 108/3=36 108/9=12
216 (2+1+6=9) 216/3=72 216/9=24
1215 (1+2+1+5=9) 1215/3=405 1215/9=135