Ho cambiato soluzione, non sapevo come fare altrimenti, usando una multiquery:
codice:
$str_queries = "SELECT * FROM tabella ORDER BY attr1 DESC LIMIT 0,5; ";
$str_queries .= "SELECT * FROM tabella ORDER BY attr2 DESC LIMIT 0,5; ";
$str_queries .= "SELECT * FROM tabella ORDER BY attr3 DESC LIMIT 0,5; ";
	
mysqli_multi_query($id_connect, $str_queries);