STRSQL="SELECT TAB1.CODICE, TAB1.DATA, TAB2.KEY, TAB3.*
FROM TAB1 INNER JOIN (TAB2 INNER JOIN TAB3 ON TAB2.KEY = TAB3.CODICE) ON TAB1.CODICE = TAB2.KEY
WHERE (((TAB1.DATA)<=#1/1/2008#) AND ((TAB3.VALORE)<>""));"
AND ((TAB3.VALORE <> ''));

singoli apici, non doppi!