T3 might have the customs, immigration and security facilities but its a bit of a dogs breakfast. When they are ready to start checking in and clearing the intl pax, the T3 sterile area has to be spilled and cleared of domestic passengers or airport visitors each time. The exercise usually takes anything from 15 - 30 minutes. International pax then get screened twice. Once to come upstairs in T3 - T4 and again to pass through international departures.