Unfortunately, residential electrical service in the United States is 120/240v single phase (technically it is split-phase, two 120v legs 180 degrees out of phase with each other plus a center-tapped neutral) in almost 100% of cases. It would be cheaper to just buy a 240v single-phase instant hot water heater than to buy a three-phase water heater and a VFD to convert the single-phase to three-phase. It’s definitely worth doing a three-phase water heater if you have three-phase electrical service though.