Welcome!
The primary resources for modding HITMAN 3 are the Simple Mod Framework, ZHMModSDK and Peacock.
Automatic mod file conversion, tools for keeping mods compatible and updated and much more.
A (still fairly experimental) modding SDK and mod loader that allows mod DLLs to dynamically alter the game with C++.
A server replacement that allows for adding contracts, items and more, integrated with SMF.
If you want to develop mods yourself, there are quite a few tools available:
An integrated modding tool for the HITMAN World of Assassination trilogy. Source (on GitHub) is licensed LGPLv3.
Includes tools for working with some additional formats not found in GlacierKit.
A few tools, including the main ResourceTool which converts between JSON and a wide variety of BIN1 game formats.
A few tools, including the main HMTextureTools which let you convert the TEXT and TEXD files to TGA.
A Blender addon for viewing and editing game assets (meshes, bone data, physics and animations).
A tool for exporting Wwise projects into the WWEM/WWEV/etc. formats of the game.