I will be arriving on SIA from Singapore in T3. Transferring to China Southern to Harbin from T2.

It is an International to Domestic transfer from T3 to T2 at Xianyang Airport. You need to pick up the luggage and go through customs clearance. Then follow the signposts to T2 to handle the boarding procedure of your domestic flight. You have enough time to catch your next flight. Don't worry. :)