I'm on three prepaid. Your mileage may vary depending on your plan (I'm using the one for iPads as a daily driver, they say you can't make calls, but you can, just the price is a bit outrageous if you talk too much).
Access Point settings:
Name: 3 (or whatever you want, I don't think it affects anything)
imobile.three.com.hk (remember to add the "i" at the beginning, that's what enables it). That alone should enable your connection. Save and try. If still with trouble, you can check the rest:
Proxy: Not set
Port: Not set
User name: not set
Server: not set
MMSC: http://mms.um.three.com.hk:10021/
MMS Proxy: 172.20.97.116
MMS Port: 8799
MCC: 454
MNC: 03
Authentication type: default,supl,mms
Bearer: Unspecified