After bud's excellent explanation, we can see that Danny is almost right.

The ground IS a parallel path for return, but most of the current will flow along the neutral wire.

If there's not a neutral wire, then..........no don't go there. Yet.

Back to that single phase feeding the house from the transformer secondary for a second.

The center tap of that transformer is bonded to ground. That gives us a zero reference. But that technically is not necessary. Your house would work fine without it. Your oven would still "see" 240 volts and your lights 120. The problem is you might have a voltage difference between some of your equipment and ground.

But transformers are not limited to one tap. This secondary could easily be tapped at 60, 120, 180 and 240 volts referenced from tap to "low" terminal.

Do I now have 4-phase power? Do I have four legs out of phase?