I assume the 3 tables have similar schemas? Try using UNION ALL to join the tables together.
SELECT ABB1.date, SUM(ABB1.credit) AS daily_total
FROM
(SELECT date, credits
FROM table1
UNION ALL
SELECT date, credits
FROM table1
UNION ALL
SELECT date, credits
FROM table2) AS ABB1
WHERE DATE >= DATE_SUB(CURDATE(), INTERVAL 7 DAY)