Make a table of the possible numbers:

 1  2  3
 4  5  6
 7  8  9
10 11 12
13 14 15
16 17 18
19 20 21
22 23 24
25 26 27
28 29 30
31 32 33
34 35 36
37 38 39
40 41 42
43 44 45
(and so on, to infinity)

You can get any number in the right column except 3 by adding 6s and 9s. So cross out the entire right column except 3. You can also add 20 to any crossed-out number and cross that number out too. So cross out everything in column two below and including 26. Finally, by the same logic you can add 20 to the crossed-out numbers in Column 2 and thereby cross out everything in Column 1 below and including 46.