If you convert to power (watts) then yes to muliply by 3, then convert back to V*A. This is important because what's going to each leg will be RMS values, and the input is a DC bus.
so a straight simplistic 60Hz conversion for 360VDC 100A: 36kW input
/3=12kW
12kW / (360V*.707) = 47A RMS per leg (67A peak)
Of course this will change based on frequency, efficiency, waveform shape, etc.. but a good enough ballpark to pick out parts.
