Create grid using default Genesis loop

gistfile1.php

<?php
/**
 * Tweaks to the post classes
 *
 * @since 1.0.0
 * @param array $classes
 * @return array
 */
function ja_post_class( $classes ) {

	if ( is_post_type_archive( 'portfolio' ) ) {
	 
		global $wp_query;

		$classes[] = "one-third";

		if ( $wp_query->current_post % 3 == 0 )
			$classes[] = "first";
	}
 
	return $classes;

}
add_filter( 'post_class', 'ja_post_class' );