Refer to the simplified schematic of the -400 below. The -800 is identical.
The packs definately output air at the lowest selected zone temperature on the ECS panel. That air then travels to the mix manifold and from there through the individual supply ducts to the respective three zones. For the two remaining zones that have a higher temp selected than the coldest, trim air is mixed into the supply duct for each of those zones to bring the supply duct air temperature up to the selected temperature for each of those zones. As NSEU posted one of the trim air valves will be closed, and the valve that is closed will be in the supply duct for the zone with the lowest selected temperature.
As you can see from the schematic trim air is hot air from the pnuematic duct that has not passed through the aircon pack. It passes through the trim air PRSOV and is directed into the respective supply ducts after the mix manifold via the trim air valves.
GB