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.