grid-posttype-archive.php

<?php 

add_filter('post_class', 'jay_grid_view_post_class');
function jay_grid_view_post_class( $classes ) {
  if( is_post_type_archive( $posttype ) ):
    $columns = 3;
    $classes[] = 'one-third';
  	global $wp_query;
  	if( 0 == $wp_query->current_post || 0 == $wp_query->current_post % $columns )
  		$classes[] = 'first';
  endif;
	return $classes;
}

add_action('pre_get_posts', 'jay_post_items');
function jay_post_items( $query ) {
	if( $query->is_main_query() && !is_admin() && is_archive() ) {
		$query->set( 'posts_per_page', '9' );
	}
}