Yes, you can use what's known as :nth-child selectors.
In this case you would use:
li:nth-child(3n) {
// Styling for every third element here.
}
:nth-child(3n):
3(0) = 0
3(1) = 3
3(2) = 6
3(3) = 9
3(4) = 12
:nth-child() is compatible in Chrome, Firefox, and IE9+.
For a work around to use :nth-child() amongst other pseudo-classes/attribute selectors in IE6 through to IE8, see this link.