You cannot extract part3.rar on its own. You must have every single part (e.g., part1, part2, part3, etc.) in the exact same folder.
If you are missing other parts, you'll need to source them from the original location where you found part 3 (likely a site like GOG-Unlocked or a similar archival community). Hitman.Absolution.v1.0.447.0.GOG.part3.rar
Library » GOG Unlocked
This specific version () is a later build of the game. Once extracted, you should find a GOG installer ( setup.exe ). Since GOG games are DRM-free , you won't need a crack or serial key to run it once the installation finishes. You cannot extract part3