Modify Post Classes on Archive Page in Genesis for columns

archive-posts-classes.php

<?php
/**
 * Adds classes to your posts for a specific page (say, a custom post type archive for instance) 
 * 
 * @author Joshua David Nelson, joshuadnelson.com
 **/

// Three Columns
function jdn_code_archive_post_class( $classes ) {
	$classes[] = 'one-third teaser'; // classes, these turn the format into three columns
	global $wp_query;
	if( 0 == $wp_query->current_post || 0 == $wp_query->current_post % 3 ) // the first and every three after that should get the 'first' class
		$classes[] = 'first';
	return $classes;
}
add_filter( 'post_class', 'jdn_code_archive_post_class' );