When inside of a loop, you can keep track of the iteration number of the loop (shown below is a simple for loop). Using that iteration number, you can calculate it's modulus of some number (number left over after an even division). If that modulus is zero, you are at an even division of whatever that number was.

So in this simple loop below, it will output something every third time through the loop:

<?php
   for ($counter = 1; $counter < 100; $counter++ ) {
     if ($counter % 3 == 0) {
            echo "<p>Every third!</p>";
     }
   }
?>