Why are there only 3 wires on this power line?

This transmission is likely 3-phase in the Delta configuration, as opposed to the Wye (Y) configuration. Delta does not have a neutral line, whereas Wye does. Wikipedia link to balanced 3-phase circuits As you can see, the Wye connection has a neutral line, whereas the Delta does not.

Long distance AC transmission is normally 3-phase without neutral.

At the receiving end, a 3-phase transformer lowers the voltage from the "several kV range" to normal household levels (maybe a bit higher if it feeds an intermediate system). For households, a neutral is taken from the output side of the transformer but, on the long-distance transmission, the transformer primary (that receives the power) doesn't need a neutral.

In my country, and probably in a lot of other countries, there is a ground wire at the top of each pylon. This ground wire is there for reasons of lightning protection. For the power transmission alone all you need is the three phases, because earth is used as reference potential. For long distance transmission there is also no strict need for a neutral wire. If there is nothing wrong, no current flows ever in the neutral line.