I arrived from Prague and landed into T3 2 days ago. I had my boarding pass for my onward flight 6 hours later from T5. I heading to flight connections and instead of going towards T5 and catch the transfer bus, I headed to T3 security at FCC. When I gave my boarding pass to the agent to scan, she told me I needed to head to T5.
I said I wanted to do some shopping at T3, she looked at my departure time and verified I had enough time. She then took my pass and checked with the security supervisor and came back and gave me the all clear.
Once in the Cathay lounge, the agents didn't say a thing about my non T3 deperature flight. So it looks like you can do if you are already in transit, but difficult to get through if your trying to enter landside T3.