I work on 744's in the middle east. The only way to keep temperatures reasonable on the ground with 400+ pax on board is to run 3 packs.
If one pack is inop for example an external aircon unit needs to be connected to the pack that's not running. There is a big difference in how much better it cools with 3 on instead of 2.
The duct pressure is automatically adjusted to keep everything running properly, and the reason you can hear and feel the difference in airflow as mentioned above between 2 and 3 packs is that with 3 packs running the pack controllers shut down the lower recirculation fans as they are not needed in that configuration.