How to output a value on every third result of a foreach statement in php?

I have a foreach statement in my app that echos a list of my database results:

<?php

foreach($featured_projects as $fp) {
  echo '<div class="result">';
  echo $fp['project_name'];
  echo '</div>';
}

?>

I would like to:

On every third result, give the div a different class. How can I achieve this?