Yeah this is the reason I didn't even take my drone when I visited last year.
There are services who will do it for you and it's not too expensive but just isn't worth jumping through all those hoops.
You're going to be bringing in an Air 3S which you purchased in a foreign country but want to use on your visit in the US?
Shouldn't be a problem. You have nothing to declare, though if you still have it sealed in the box, customs agents might...