The RN Sea Kings (HU5 btw, not HAR5) have shorter range than the 3/3A due to their having a different fuel system which fits around the hole in the floor through which the sonar was dipped. The 5 & 3 share the same AFCS (stab and basic autopilot) while the 3A is comparitively far more swept up, allowing it to 'fly itself' to a much greater degree. The 5 has a different radar to the 3/3A which in itself has similar capabilities, but the 3/3A benefit from the fact that the radar picture is processed and displayed on a 'TV' screen rather than an 'old school' screen. The 3/3A are all fitted with MSS turrets (providing IR and TV search capabilities) whereas
AFAIK the 5 is yet to get this capability across the fleet. The 3/3A have far more comprehensive IFR nav kit as compared to the 5 which didn't need this in its original role. The layout of the cabin in the 3/3A is more optimised for SAR, so working with stretchers is easier than in the 5. Finally the 3/3A have far superior external lighting for night jobs.
That said, they all fly in a pretty similar fashion (3A pilots will disagree here), and they are all very effective, if aged, SAR cabs.