WP_Query using filter posts_groupby, but getting last post from author

Using this:

$col = 'posts_per_page=4&author=1,2,3';

		function filter_authors($groupby) {
		  global $wpdb;
		  $groupby = " {$wpdb->posts}.post_author";
		 return $groupby;
		}
		add_filter('posts_groupby','filter_authors');
		$the_query = new WP_Query( $col );

The filter works very well, but the results are ASC, I need it to be DESC. Is there some hack to it?