JFK-SEA is not a 3 class flight; the only, regularly scheduled/advertised domestic transcons on 3 class aircraft are the new A321 (32Bs
https://www.seatguru.com/airlines/Am...s_A321_new.php) (you may be able to luck into a international bird on a positioning flight to a follow-on international flight, e.g.
https://thepointsguy.com/2017/10/aa-...estic-oct-dec/). Even if you are on an internationally configured multi-class plane (e.g. actual international Business or F) and you are not in Coach, the service (outside of the flagship JFK-SFO/LAX routes
https://www.aa.com/i18n/travel-info/...ontinental.jsp) are usual domestic F.
So, it's domestic F on a domestic aircraft. There are various reasons why it might be classified as business (either rev/yield management, or the flight number continues ex-SEA internationally), but it's domestic F service. Nothing fancy (I've flown this route before in F).