Between 3-4 pm on Sunday there is a bit of Air Transat flights back from vacation destinations so there could be a crowd in front of you at arrival processing.

BA's arrival baggage should be faster unloaded than the Air Transat flights.

After picking up bags and clearing customs, you would be at one end of arrival hall, head towards the other end and take escalator/elevator up to departure level and you will be at US departures. Most arrows will direct you to head upstair as soon as you enter arrivals hall but arrival level is less congested than the departure level.
Check in is by kiosk and your bags are tagged, then dropped off at US bound flight conveyor. If you are running late, past 1730 let anyone with a AA uniform know and they will try to expedite you through security and US immigration.
Sunday afternoon should not be too bad for US departure at T3.

Sorry I forgot you need to go through flight security before US CBP immigration.