On my Amazon account, I can download my purchased books as AZW3 from the following page: https://www.amazon.com/hz/mycd/myx https://www.amazon.com/hz/mycd/myx (

On my Amazon account, I can download my purchased books as AZW3 from the following page: https://www.amazon.com/hz/mycd/myx https://www.amazon.com/hz/mycd/myx ('Manage Your Content and Devices'). (As I understand it, AZW3 is mostly the same thing as EPUB3.) (Either that, or the files I download from there aren't actually AZW3 files but just KFX files with an .azw3 extension.)