The suffix part3.rar indicates that the game has been split into multiple smaller files for easier distribution and storage. In the era of digital preservation and limited bandwidth, splitting large game directories into chunks (typically 1GB to 5GB each) serves several purposes:
Multi-part archives are the standard for community-driven preservation efforts, allowing files to be hosted across various mirror sites. Conclusion Killer.is.Dead.Nightmare.Edition.GOG.part3.rar
Additional DLC content, including extra missions and "Gigolo Glasses." The suffix part3