Something like this should work:
SELECT DATE_FORMAT(DATE_ADD(`expiry_date`,INTERVAL 3 DAY), '%d %m %Y' )
FROM `plan_cbsubs_subscriptions`
WHERE `user_id` = '[user_id]';
to subtract 3 days:
SELECT DATE_FORMAT(DATE_SUB(`expiry_date`,INTERVAL 3 DAY), '%d %m %Y')
FROM `plan_cbsubs_subscriptions`
WHERE `user_id` = '[user_id]';
you can find more information here: MySQL DateAdd