add-col-classes-to-entry.php

<?php
//don't add

/* 
 * Add Genesis one-third (3 col) class to the article.post in archives 
 * in the main query. Uses Genesis specific filter 
 * instead of WordPress's post_class filter
 * to do the same thing 'cause why not!
 *
 */
function themeprefix_add_column_grid_entry_archive( $attributes ) {
 
	global $wp_query;
  
  	//is the main query and is not singular
	if( $wp_query->is_main_query() && ! is_singular() ) {
 
		$attributes['class'] .= ' one-third';

 		if( 0 == $wp_query->current_post % 3 ) {
			$attributes['class'] .= ' first';
		}
	}

  	return $attributes;

}
add_filter( 'genesis_attr_entry', 'themeprefix_add_column_grid_entry_archive' );