Add a class to third, sixth, 9th, post

I am using this code for displaying posts within a certain category.
<?php if (have_posts()) : while (have_posts()) : the_post(); ?>

li show something li
<?php endwhile; else: ?>
<?php _e(‘Sorry, no posts matched your criteria.’); ?>
<?php endif; ?>
For styling purposes i would like that every third displayed post to have <li class”example”> instead of
li . Something like oddcomment function.

Thanks in advance!