Problem with a third run-way is that it's going to be another major land-fill, and left-wing environmentalists are going to have a field day with this, both in protests and in the courts. Not sure how powerful they are in HK but they are quite powerful in US and a reason US is lagging behind in major infrastructural projects.

But I am always surprise how efficient HKG can be with only two runways, when you think about the amount of take-offs and landings it has as a major East-Asian hub.

But yeah, if HKG is going to be competitive, especially with the rise of Shenzhen, Zhuhai, and Guangzhou, they should really position themselves well with a third run-way. As it is easier and easier to transport to Shenzhen they have lost many intra-China pax to it, and the future bridges that connects HK-Macau-Zhuhai will only make the problem worst.