я думаю что-то похожее на это должно быть:
SELECT u.login AS title
,u.user_id AS id
,n.title AS npc.title
,n.npc_id AS npc.id
FROM users AS u
INNER JOIN game AS g
ON u.id = u.user_id
INNER JOIN npc AS n
ON g.id = n.npc_id
AND g.type = 'npc'
WHERE g.type = 'user'
AND g.loc = '".$user['loc']."'
AND u.user_id!= '".$user['USER_ID']."'