MYSQL query is getting data exactly as I want.But ORDER BY clause is not working.  
But when I removed the GROUP BY from query then ORDER BY clause is work.
SELECT MESS. * , US. * , LF. *
    FROM messages MESS
    LEFT JOIN users AS US ON US.id = MESS.to_id
    LEFT JOIN label_infos LF ON LF.user_id = MESS.to_id
    WHERE MESS.frm_id =27
    OR MESS.to_id =27
    GROUP BY US.id
    ORDER BY MESS.id DESC
 
    