Select posts of onyl one category

Hi,
in this query (of the xml export function):

<?php if ($post_ids) {
          global $wp_query;
          $wp_query->in_the_loop = true;  // Fake being in the loop.
          // fetch 20 posts at a time rather than loading the entire table into memory
          while ( $next_posts = array_splice($post_ids, 0, 20) ) {
            $where = "WHERE ID IN (".join(',', $next_posts).")";
            $posts = $wpdb->get_results("SELECT * FROM $wpdb->posts $where ORDER BY post_date_gmt ASC");
              foreach ($posts as $post) {
            setup_postdata($post); ?>

how to select only posts of the category who have the ID=3 ?

++