PC power supplies can be thought of as being a bridge rectifier (at 230V setting) or a voltage doubler rectifier (at the 120V setting), feeding a big capacitor that has a more or less resistive load across it. The rectifier diodes only conduct at the very peak of the input AC voltage to "top off" the capacitor to replace what the resistance drained off between voltage peaks. This makes for current spikes happening at the input AC voltage peaks, and no current draw at all between peaks. These spikes last for about 5% of the cycle time of the 60Hz waveform cycle. Thus there is *NO* overlap from phase to phase. PCs on phase A draw their current spikes at the peak of A from A and the neutral. Then likewise for phase B, and then C. The neutral thus sees 3 times as many spikes as any one phase wire does, and over time will get as hot as the same guage of wire carrying 3 time the current of any one of the phases. You just need to think in terms of the time domain to see this.
Heard it said that the European equivalent of UL and/or FCC will soon require that switching power supplies draw current off the lines like a purely resistive load would. This technology exists; it just costs more....
