Thief 3: Deadly Shadows [cheats] Go to the Thief - Deadly ShadowsSystem directory
Copy the DEFAULT.INI file for backup

Then Open the DEFAULT.INI with a texteditor like notepad.
Scroll to the [Difficulty] section. or use the find option ;)

AI visual acuity multiplier - how well AIs can see
AI auditory acuity multiplier - how well AIs can hear
AI tactile acuity multiplier - how well AIs can feel
AI hitpoints multiplier - all AI starting hitpoints will be scaled by this
AI-to-player damage multiplier - all damage applied to the player by the AI is scaled by this

change the multiplier for see/hear/feel/hitpoints//damage to whatever u feel like, (0)zero usually does it hehe.
As u can see the settings are for easy/normal/hard or expert level. If u want u can change them all, but its not necessary just remember which one u changed.

-from Dev