It would make sense for it to operate the same as it does for T5B, in that departing pax leave on the 'outsides' and arriving pax on the 'insides'. Trains could then run straight through and then back again, as they effectively do with B now. The issue comes with C to B or B to C connections, unless they expect everyone to go to A and then back again.
You cant do that and run 3 trains. They have to be able to pass somewhere. It will still be arriving on inside and departing on outside (or vice versa in t5a). The layout in T5B station remains the same but only 1 side of the 'landside' (not UK sterile) central platform will be used. The trains heading to T5C can only open doors on the out/airside.
Dont know how they plan to run transfers but B to C should be possible by clearing security in B and boarding "sterile" train to C or by boarding "non sterile" train to A from C getting off at B and clearing security there. Might be a bit complex though!