I'm not really sure what the point of the 3rd runway is unless there can be two simultaneous arrivals or two simultaneous departures. That seems impossible without a major revamp of the airspace's departure and arrival corridors. It seems to me that the existing two runways are not utilized to capacity now. Whether this is due to airspace issues, poor ATC, or a combination of both is unknown to me.
I do believe that HKIA would be able to move more traffic than it does currently if an ATC team from LHR or the US came in to consult and improve procedures. That said, HKIA has a higher concentration of widebodies (more separation required) than most places and many of the operators have extremely poor English skills. So, ATC's ability to get aircraft to do what they want is lessened. I actually think HK ATC has improved in recent years, but it seems there is more to be done.