If the train station at T5C is designed carefully, the trains could run in a circular operation i.e.

T5A - T5B - T5C - T5B - T5A

At T5A, airside passengers board all carriages.

Those that need to alight at T5B do so. Connecting passengers to B&C gates from T5B can also board at T5B through the same platform that incoming T5A passengers have got off from.
This train continues to T5C where all passengers get off.
A new set of arriving passengers board at T5C. This train then stops at T5B where more passengers join, and in addition some passengers can get off (to the same platform as those joining passengers) who are connecting to T5B and T5C (assuming there is no security in T5C). This train continues to T5A where everyone disembarks.

Assuming a circular method of operation, three trains can operate in a circuit at any time.

This is surely the most efficient way to run a transit system?