That's a higher voltage power line (in the low kV range) with just the three phases. They're relying on the load being approximately distributed between the three in the downstream distribution network to obviate the need for any return wire. If you move forward one step on that road you will see the next pole has a lower set of wires which will be at 230/240 volts. On these it looks like there are four wires, they're probably doing without the earth wire in this rural area and just tying to earth periodically.