While specific documentation depends on the source (such as Nexus Mods or a specific Discord community), files with this naming convention generally follow these steps:

The file is the third segment of a multi-part compressed archive, typically associated with a large digital download such as a game modification (mod) or a community-made content pack.

If the file is corrupted or won't open, verify the if the provider supplied one. A mismatch usually means Part 3 was an incomplete download.