Incidentally the reason I reckon T3 for Cathay is that they're ONEWORLD and ideally might join BA and AA under the same roof
Makes more sense to be with BA/AA, but then they used to be in T2, and the recent pax flights used T2.
Also worth considering, AA codeshare with EY, but EY are in T1 with AA in T2.
This is one of the downsides of codeshareing, it means numers are all oer the shop.
For example, AF is part of KLM, but AF in T3 with KLM in T2. If KLM moved to T3, then you have the DL codeshare with KLM still in T2.
I suppose with CX, when its finally announced we will know the terminals.