Having 3 identical hydraulic systems would increase the aircraft weight by a lot. Heavy duty hydraulic pumps, wide pipes to allow for appropriate flow of hydraulic fluid, etc. Also, if you would get a catastrophic failure on one of the (relatively unimportant) flight control surface, you could loose all hydraulic fluid and thus systems - highly undesirable.
The hydraulics of the aircraft are usually designed with redundancy in mind. So the loads are split between smaller hydraulic systems, so the failure of one or two hydraulic systems doesn't affect aircraft ability to fly.
On top of all, hydraulic failures are pretty rare, so the odd return to base or diversion is not really an issue.