Files that contain the .img3 file extension are normally associated with encrypted container image files. These container files store encrypted images and are commonly used for iPhone and iPod devices. IMG3 files were sometimes used to "jailbreak" an iPhone that a user had purchased. These files can be converted to more versatile file formats using the proper conversion software application.
The IMG3 file format was created to replace the older IMG2 file format. It was used on iOS 2.0 and onward.
The IMG3 file format no longer seems to be supported and has been replaced with more current file formats, such as the S5L file format.
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