To be divisible by 3, the sum of the digits must be
divisible by 3.  My steps were to write them out 
making sure their digits have sum 3,6,9,10,12,15 or 18


 1. 102
 2. 111
 3. 120
 4. 123
 5. 126
 6. 132
 7. 162
 8. 201
 9. 210
10. 213
11. 216
12. 222
13. 231
14. 261
15. 300
16. 303
17. 306
18. 312
19. 321
20. 330
21. 333
22. 336
23. 360
24. 363
25. 366
26. 600
27. 603
28. 606
29. 612
30. 621
31. 630
32. 633
33. 636
34. 660
35. 663
36. 666

Edwin