A third runway is imperative to the future of HKG, especially if you see the kind of growth in China. Shanghai has a two capable airports with PVG and SHA, and more and more people are now electing to use airports close to HKG even if their destination is in HK. With land transport becoming easier and other Canton airports improving, not building a third runway would let other hubs catch up to us.
As for environmental concerns, there is always a debate between economic benefits and environmental damage, and without knowing the full report I can't comment. Anyone know the extent of the damage?