This code adds a unique class to every first/second/third multiple in the WordPress loop:
<?php
$style_classes = array('first','second','third');
$style_index = 0;
if (have_posts()) : while (have_posts()) : the_post(); ?>
<div <?php $k = $style_index%3; echo "class=post $style_classes[$k]"; $style_index++; ?>>test</div>
So the first post has a class of first, the second a class of second, the third a class of third, and then it resets with the fourth post having a class of first, the fifth post a class of second, etc etc.
Is there a way to also make only the first three posts have an additional class called “special”?
You should not compress together so much code into one line.
Instead, make one statement per line, that keeps you flexible to add stuff, like your special class: