On my SAT II, I received a 800 on Math II, a 770 on Chemistry, and a 760 on US History. I received a 5 on the AP US History exam. Should I still send my US History SAT II score, or is it too low and a 5 would look better?
You can self report the AP score. Official AP score reports are not required for admissions, and AP scores carry little to no weight in the admissions process.
As far as the Subject tests, you can send all three or just the top 2; I doubt the third one will move the needle at all, though. But congrats on the great scores.