I ran into one of those systems in a small
commercial garage in Aberdeen md. 3 wires in what looked like a single phase breaker box,
2 wires on the normally hot posts ,the third
on the neutral . measured 240 across any two. Thought this is odd, followed the service cable (2wire with wraped neutral) outside, it's connected to a triplex (2 insulated and a bare neutral), followed it several pole spans down the street to a pole with 3 small transformers. That was the first I had seen that ,and I don't remember that set up from college.