Du kan JOIN til en "tabel" oprettet fra et undervalg som dette:
SELECT t1.user_id, t1.user_name, t2.department, t3.computer_name, t3.logon_date
FROM table1 AS t1
LEFT OUTER JOIN table2 AS t2 ON t1.user_id = t2.user_id
LEFT OUTER JOIN
(SELECT user_id, computer_name, MAX(last_logon_date) AS `logon_date` FROM table 3 GROUP BY computer_name) AS t3 ON t1.user_id = t3.user_id