Delete records older dan 3 hours

DELETE FROM `my_table` WHERE `created_at` < (NOW() - INTERVAL 3 HOUR);

Delete records older then 3 days

DELETE FROM `my_table` WHERE `created_at` < (NOW() - INTERVAL 3 DAY);

Delete records older then 3 months

DELETE FROM `my_table` WHERE `created_at` < (NOW() - INTERVAL 3 MONTH);