Your Z3 might support AT&T LTE.
If it has a model number D6603 or D6643 it should be fine.
All models should support AT&T 3G and 2G.
To find the model number and name of your Xperia™ device
From your Home screen, tap the Application screen icon. Find and tap Settings. Tap About phone/tablet > Model number.
https://www.lfgss.com/comments/13301007/
@hippy started
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Your Z3 might support AT&T LTE.
If it has a model number D6603 or D6643 it should be fine.
All models should support AT&T 3G and 2G.
To find the model number and name of your Xperia™ device