difference in the amount of electric used in a 3 phase water heating element over a single phase

It takes same power (Kwh) to heat water, no matter which element.
So difference would be how much power is saved using 3-phase, and how much power is saved using higher voltage.

Typically, 3-phase runs with higher efficiency, and delivers more power with less heat loss vs single-phase.
Using higher voltage, such as 480 volt (vs residential 240 volt) delivers power at higher efficiency than single-phase 240 volt.


More detail:
A 3-phase water heater has varieties

1) Unbalanced 240 volt 3-phase uses ordinary single U element ... same element as residential water heater ... and these heaters are probably same efficiency as residential heater, maybe slightly higher because it's 3-phase ... so in this case there is probably small difference.

150 to 2,500 gallons
6100 watt element
 2) Unbalanced 480 volt 3-phase has ordinary single U element, but element is rated 480 volt.
Higher voltage delivers more watts of heating power with less amperage ... so less heat loss on wire ... so slightly higher efficiency than residential heater.
Standard voltages include 208, 240, 380, 400, 415, 480, 600 volt single or three-phase
l 66, 80, 120 models can only be ordered 277V, 240V, or 480V

3) Commercial 3-phase balanced 480 volt water heaters have 3U element ... a genuine 3 phase heater ... and the element is more efficient since it's higher voltage plus 3-phase delivers power with higher efficiency.

So a 3-phase 3U element that runs on 480 volt will use same Kwh hour of power as residential single-phase single-U element to heat water, but since higher voltage 3-phase runs with higher efficiency, then 3U element 480 volt 3U element can deliver with higher efficiency than single phase single-U element..