As others have said taxiing in with number 3 shutdown was a common SOP for many years at various airlines, at our outfit shutting down number 2 as well came along afer I left the fleet..
but I am guessing there are no PTUs or Electric Hyd pumps to maintain systems when engines are shut down?
Don't remember losing any hydraulic system with number 3 shutdown - Now I might be wrong 'cos it's "been a while" so this is a strictly as I recall it but I
think even with 2 and 3 shut down their respective hydraulic systems will still be powered by the respective demand pumps ( Air Driven Pumps using Bleed air from the main Bleed duct, rather than using Bleed air of it's "own" engine...) .
As Tower dog mentions there was also an electric pump on system 4 for ground use (e.g during push back).