Apply class to every third list item? [duplicate]

add a simple counter and conditional check:

$pages = get_pages('child_of=10');
$counter =  1;
if ($pages) {
    echo '<ul class="projectthumbs">';
    foreach ($pages as $page) {
        if ($counter == 3){
            $class=" class="YOUR_CLASS"";
            $counter = 1
        }else{
            $class="";
            $counter = $counter +1;
        }

        echo '<li'.$class.'><a href="'.get_permalink($page->ID).'">';
        echo get_the_post_thumbnail($page->ID);
        echo '<span class="projectthumbtitle">';
        echo get_the_title($page->ID);
        echo '</span>';
        echo '</a></li>';
    }
    echo '</ul>';
}