Código PHP:
SELECT i.num_serie,i.nom_comercial FROM Impressora AS i,Compra as c WHERE
i.num_serie = c.num_serie
AND
c.CIF_client IN ( SELECT cl.CIF_Client,c2.data_compra FROM Client AS cl , Compra AS c2 WHERE
cl.CIF_Client = c2.CIF_Client AND c.pais='EUA' AND c2.data_compra IS NOT NULL
GROUP BY cl.CIF_Client HAVING COUNT(c2.data_compra)>3
GRACIAS !