Share your experience!
That's something you have to take to AT&T, as its up to them to carry or not a brand.
The Xperia Z3 D6603 will work fine with AT&T 3G and 4G LTE bands.
I don't understand your question. but you can use the Z3 D6603 with AT&T, just like any other phone that's compatible with AT&T
I don't have this phone but I know someone that does and use it with AT&T, it works great.