T3 would be greatly relieved if the remaining non-aligned airlines, exc. VS moved to T4 as was once mooted. The handful that have will have made only a small difference. I don’t know if further shifts to T4 are contemplated, given T3 was meant to be the OneWorld + VS terminal. Indeed Aer Lingus was also meant to shift to T3.
The shift of some BA flights to T5C in 2010 will help though.
Then it will be the remaining *A airlines (AC, TG, SQ, NH, SK) to the new T2 which will provide the final relief of T3 (which presumably will have either another facelift or by then will T6 be contemplated?).