I never heard of a "3-phase electric fence" either. But if in fact such a device exists the 3-phase part of it would be just the input to the charger. The output would most likely still a pulsed DC as with any other common fence charger. IIRC, "AC" pulsers are a thing of the past, but in fact pulsed DC is a form of AC, so it gets too complicated for this feeble mind to sort out. (To be sure, last time an electified fence knocked me on my a$$, I wasn't too concerned whether it was AC or DC!) Now if you're talking about a security fence vs. livestock control, well that could be a whole different story!
Myth Busters had an interesting show on this topic. They concluded one could not be shocked by peeing on an electric fence simply due to the fact that the pee stream is itself "pulsed" - a series of individual drops vs. a continuous flow which could not provide continuity back to the "source". Their final mock-up was hillarious, about as close as you could get to the real thing. Regardless, I wouldn't want to test it out myself...!
You would have to get real unlucky for my electric fences to hit you from peeing on them. I do know that my bucks often back up into it when sparring and often the bigger of them touches the fence with his meat and two veg.... its amazing how high they can jump when that happens.