1. for 研究岗, go to the most solid program, or the one with best academic reputation.
2. for 一般事务性岗位, go to the one with the best general business reputation. (it is the reputation in the industry, instead of relatives count)
3. I don't think HKUST is way better than the other two, indeed just they are same tier universities. but business, economics are what HKUST is good in.
So actually you wont go too wrong if you choose HKU, or CUHK instead of the HKUST one. Difference is small, and likely you will get similar interviews......it is like a comparison between scoring 90, 89 and 88....
4. HK is small, location doesn't matter much, if not at all.