Email addresses being set to on-hold bounce due to Away responses
You can change the hold_threshold in civicrm_mailing_bounce_type table for Away to your own choice. By default it is set to 30. Not sure why your system is set to 3. 

I dont think if there is any way to discard old bounces unless you delete from civicrm_mailing_event_bounce table. Probably good idea to write a script that runs on cron job to delete the entries of certain bounce type and X days old.

Cheers

Pradeep