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.
Library » GOG Unlocked
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. Hitman.Absolution.v1.0.447.0.GOG.part3.rar