AA doesn’t have all that many flights, at least not compared with BA, so it’s not huge. The previous plan also included making better use of the T5D stands. But if I were a betting person, I’d say it would be the ‘holiday’ type routes that would move to T3. There are some BA flights that have hardly any connections, unlike (say) the AA ones, where it’s over half on some rotations, and nearly all to/from BA.