HK is a pretty small place with few "real" tourist attractions...
There's not much to see in Kowloon outside of TST for the tourist. Ditto for Lantau except for the Big Buddah. New Terrortories?? you gotta be joking. I've been here since 2000 and have never found any reason to go the New Terrortories for sightseeing.

3 days is more than enough. You could easily cover the major sites in a day. The Big Buddah would be half a day before your flight. Leaves a day for shopping or Macau. If neither of those options sound enticing.. well.. you'll pretty much be outta things to do.

Honestly, I would go to Phuket. Hong Kong is much less a tourist destination then a commerical center. Personally, I would never come here as a tourist. If you have to go to Singapore, I think it'll really be nice for you to spend some time relaxing in Phuket instead. If you've been there before how about Bali?