Select rows from two joined table that not in a third table
SELECT A.*, B.*
FROM A 
JOIN B ON A.user_id = B.user_id
WHERE NOT EXISTS ( SELECT NULL
                   FROM C
                   WHERE C.user_id = B.user_id )


or

SELECT A.*, B.*
FROM A 
JOIN B ON A.user_id = B.user_id
LEFT JOIN C ON C.user_id = B.user_id
WHERE C.user_id IS NULL