To calculate the load for a Single Phase Panel I understand you add up all the individual loads in Watts and divide by the system voltage. This will give the Amps for the phase conductors and allow you to size them appropriately.
With a Three Phase Panel I suspect you would take the same action up to the last step where you would divide the total Wattage by system voltage times 1.73. Total Watts / (voltage * 1.73) = Current carried by each Phase Conductor.
I understand that is a simplification and does not take Power Factor into consideration and assumes the panel loads will be evenly balanced, but is the basic process correct?