003 file extension is associated with various file archivers. File archivers are dedicated software tools for archiving files and folders, usually using some type of compression algorithm.

003 file extensions is added to files that are a part of multi-volume archives. Each part of multi-volume archive is given a numbered extension (001, 002, 003, etc.). 003 extension indicates that given files is the third part of such archive. In order to unpack a multi-part archive all parts of it must be present in on disk. WinRAR or 7-ZIP are one of the most popular tools that use this multi-volume archive file numbering scheme.