.azw3r is Amazon Kindle metadata
Key Features: .AZW3R File Extension
What is an .AZW3R file?
The azw3r file extension is mainly related to Amazon Kindle e-books and used for some sort of metadata file for standard azw3 file.
Typically located within .sdr subfolder, these file have the same file name as the main *.azw3 e-book.