If I had 3 hours at HKG I'd be munching in either of the Cathay First Class lounges.

Not sure I'd rush into the city with just 3 hours.
I've always been happy with the shopping at Singapore if I have time to kill. Seem to always buy something there to burn off my remaining currency. Came home with a travel alarm clock this year, and rechargable batteries the time before, and that was after visiting the shared BA/QF lounge.