// Вывод 3-х рандомных файлов
$req = mysql_query('SELECT t1.id, t1.name, t1.screen 
FROM download AS t1
JOIN (SELECT (RAND() * (SELECT MAX(id) FROM download)) AS id) AS t2
WHERE t1.id >= t2.id ORDER BY t1.id ASC LIMIT 3;');
while ($arr = mysql_fetch_assoc($req)) {
  echo '<img src="../download/screen/' . $arr['screen'] . '" alt="' . $arr['screen'] . '"/>';
  echo '<a href="../download/?act=down&amp;id=' . $arr['id'] . '">' . $arr['name'] . '</a>';
}