codice:
SELECT u.id, u.nome_arte, u.foto, u.hits, u.operante, u.sesso, 
u.nascita, u.city, x.id, y.id, c.nome AS nomesottocat
FROM users AS u 
INNER JOIN users_cat AS c ON u.id_tipo=c.id 
LEFT JOIN users_cat AS x ON c.parent=x.id  <-- QUESTO!!!
LEFT JOIN users_cat AS y ON x.id=y.parent 
WHERE y.parent='".ID_ARTISTI."' AND u.attivato='1' ORDER BY u.hits DESC