The APU doesn't drive a hydraulic pump directly but I think that your assumption that the APU doesn't provide hydraulic redundancy is incorrect.
When running the APU is pushing out lots of electrics and lots of bleed air that feed various systems, including the three hydraulic systems that have a combination of either electric or bleed air driven demand or primary pumps. If you run the APU in flight, say in the event of a single engine failure, I'd say the APU is providing hydraulic system redundancy, if needed, via the air driven and/or electric driven pumps.... ( you don't deploy the RAT just for one engine out).
As evidence of the APU s capabilities it is worth bearing in mind that it is also certainly powering the hydraulics, again indirectly via electrics/bleed air, during the initial stages of pushback, before any engines are running.
In most circumstances the RAT will provide essential but limited hydraulic power but it is really a fourth in line backup for hydraulics, after the two engines and the APU.
As an aside whilst the 777 APU is fairly powerful I've never heard it ever described or considered as a third engine.
Last edited by wiggy; 10th Apr 2017 at 09:11.