To update my post on the madhouse for transit passengers at T3, they've now closed the small, separate security checkpoint for transit passengers and instead just thread transit passengers into the main security checkpoint. That seems to work well and has eliminated the madhouse bottleneck. Don't know whether it's a permanent or temporary change, though.
It was working fine last week.
When transiting to a domestic flight, if you have our boarding pass and your luggage is checked through, you do not need to pass immigration and go to the main hall. You can access domestic gates from the luggage carousel area.