Use the formula for parallel resistors to find the resistance of all the legs in parallel. Use this resistance along with the load to find the voltage drop for the parallel sets of conductors. Use the voltage drop and the resistance of each individual conductor to to find the current on that conductor.
It is my understanding that the inductance of an installation like this is often the biggest problem. To minimize its effect, you should group the conductors in the wireway with all 3 phases and the grounded conductor, if used, in each group.
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