What voltage input/output and power output are you talking about???
Most likey you would use a high freq transformer to boost to just over the RMS value of the output. The harmonics are better if the output's not a pure square wave but lingers at zero for awhile on each zero crossing. Thus the rail needs to be a bit over the RMS output value to compensate.
You might create a dual supply, in which case you'd have a fixed common on the output and flip between supplies, or single supply, in which case you'd flip the hot/common wires against the supply.
I think the same strategy works for 3 ph, but I don't know for sure.