Given the odd ratio involved with three phase power, the only practical way to make a resistive element appliance work on either single phase or three phase is to have 3 elements that provide 1/3 of the heating. They are wired parallel for single phase, and star (wye) for three phase (including possibly without the neutral attached). Dual voltage single phase (as in North America) has the option to use 2 elements in either parallel for the lower voltage or series for the higher voltage (and the neutral is not needed unless its 2 phases of a 3 phase system). So there are a lot of possible wiring options, and a neutral is no always necessary. But having elements designed for direct phase to phase connection (whether three phase or single phase) sacrifices alternatives.
