You can count by three's by adding three to the previous number.

The numbers that you would count if you started with 0 and counted by threes would be:
0, 3, 6, 9, 12, 15, 18, 21, 24, 27, 30, 33 and so on.

The numbers that you would count if you started with 1 and counted by threes would be:
1, 4, 7, 10, 13, 16, 19, 22, 25, 28, 31, 34 and so on.