Ensure you have every numbered file from the set in one directory. Use 7-Zip: Right-click the .001 file.
Select 7-Zip > Extract Files... or Extract Here . The software will automatically detect the other parts and combine them back into the original content. mm_003.7z.001
Do you have the of this archive, or are you seeing an error when trying to join them? Ensure you have every numbered file from the
This filename structure, , indicates the first segment of a multi-part compressed archive created with 7-Zip . mm_003.7z.001