Intruder - one of the reasons for killing 1 pack on the ground is that during days with high humidity and temps, the fwd cabin tends to rain from the hatrack grilles and if you have too low a cooling demand dialed in, you may even hear what sounds like rats having a dance party behind the left hand side upperdeck stowages as ice pellets make their way through the cockpit delivery ducts. Apart from the above, it needs to be a VERY uncomfortable day before 2 packs on a B744 can't cope so why burn gas for the sake of a 3rd pack ? Have a look at your pack mass flows and you'll probably find that the total of 2 versus 3 isn't that much different. Look after the cents and the dollars look after themselves in this case as you save fuel as a bonus.
