Add class to every third post?

You can try it with a counter. Here is a very simple loop that gives every third post a class of “third-post”;

<?php $counter =0; ?>
<?php while (have_posts()) : the_post(); ?><!-- begin of loop -->
<?php
  ++$counter;
  if($counter == 3) {
    $postclass = 'class="third-post"';
    $counter = 0;
  } else { $postclass = ''; }
?>
<div id="post-<?php the_ID(); ?>" <?php echo $postclass; ?>>
<?php the_content('Read the rest of this entry ?'); ?>
</div>
<?php endwhile; ?><!-- end of loop -->

Thanks a bunch!