That was my opinion as well, however my more senior colleagues insist that running 3 at once for more than 10 minutes or so puts undue strain on the APU and can cause it to fail.
Part of the issue I'm sure is that we operate predominately freighters and pack 2 is usually kept off since 1 & 3 provide the airflow to the upper deck anyway.
Last edited by RandomPerson8008; 25th Sep 2014 at 11:03.