Change of Airline and changing Terminal 3 to Terminal 2, Beijing

Hi: we arrive by Air China, Terminal 3, at 12:35 and fly to Tashkent with China Southern from Terminal 2 at 16:15. We are advised that we have to clear immigration, pick-up our luggage, clear customs and then shuttle to Terminal 2 to check-in. I hope someone can advise the process, timing and any other information. Any info is appreciated.