SELECT * FROM Customers3 WHERE (3cstfirst, cstlast, cstphone) NOT IN
   (SELECT cstfirst, cstlast, cstphone FROM Customers1)
AND cstfirst, cstlast, cstphone NOT IN
   SELECT 2cstfirst, cstlast, cstphone FROM Customers2)

Fixed the missing (

SELECT * FROM Customers3 WHERE (3cstfirst, cstlast, cstphone) NOT IN
   (SELECT cstfirst, cstlast, cstphone FROM Customers1)
AND cstfirst, cstlast, cstphone NOT IN
   (SELECT 2cstfirst, cstlast, cstphone FROM Customers2)