Wrap with a div
From http://stackoverflow.com/questions/8947878/php-loop-add-a-div-around-every-three-items-syntax
<?php
$i = 1;
//added before to ensure it gets opened
echo '<div>';
if ( $wp_query->have_posts() ) : while ( $wp_query->have_posts() ) : $wp_query->the_post();
// post stuff...
// if multiple of 3 close div and open a new div
if($i % 3 == 0) {echo '</div><div>';}
$i++; endwhile; endif;
//make sure open div is closed
echo '</div>';