003.7z Official

When a user creates a large compressed archive and chooses to split it into smaller pieces for easier sharing or storage, 7-Zip generates a sequence of files. The extension .003 indicates that this is the of a split archive. Key Characteristics

It contains a portion of the compressed binary data. The 7-Zip software (or compatible tools like WinRAR or PeaZip) uses the index provided in the .001 file to stitch these pieces back together into the original file. 003.7z

“When 7z opens a 7z archive it would just check if it ends with '. 00x'... then it would know that it is a part of a split archive and could iterate true all split files.” SourceForge · 10 years ago When a user creates a large compressed archive