Add class every 4th,5th,6th,10th,11th,12th etc...

Hi, i have an unlimited grid of divs in rows of 3. How would be the best way to achieve adding a class to the 4th, 5th, 6th, 10th, 11th, 12th, 16th, 17th, 18th..... etc... div?

i don't believe this is achievable with the:nth selector?

any help appreciated.