tabella1=rifstruttureesterne
tabella2=rifstruttura
tabella3=rifsede
SELECT Tabella1.*
FROM (Tabella1 LEFT JOIN Tabella2 ON Tabella1.id = Tabella2.id) LEFT JOIN Tabella3 ON Tabella1.id = Tabella3.id
WHERE (((Tabella2.id) Is Null) AND ((Tabella3.id) Is Null));