I think American really want to be in T5 but in normal times, as per summer 2019 they have 21 heavies per day and even BA were split between T3 and T5. Now I know it's all metal neutral and Iberia have been co-located at T5 for years but with BA Gatwick drastically downsized until next summer at the earliest, will BA not want to run one single terminal operation rather than split ops in T3? BA also ran long haul flights out of T3 with MIA, DEN, PHX and LAS alongside ACC, NBO,CPT and YVR.
Options :
AA split ops T3/T5 with JFK plus key others in T5, remainder in T3
AA move everything into T5 leaving BA to move some of their own flights to T3 or LGW
Neither is tidy but I suspect HAL will move all of AA into T5 as BA are moving to T8 at JFK and there's give and take on both sides.