A three phase UPS would have nothing particularly special. There would just be 3 DC to AC inverter circuits coordinated from the same timing or waveform generator. It wouldn't really matter much of it was a square wave output or sinewave. Nor would it matter if it is line interactive or online dual conversion.
I suppose some UPS designer might go cheap and use just 2 phases at 90 degrees and try to make a "tee" source instead of a normal "wye" source. I'd stay away from those.
