Now, that being said, you'll want to find the Thief 3 directory and go to the System Folder. (Default is: C:\Program Files\Thief - Deadly Shadows\System). Look for default.cfg. Open up this file in your normal text editor and look for this:

[Difficulty]
-- THIEF 3

AI visual acuity multiplier - how well AIs can see
Difficulty_AIVisual_Mult_Easy=0.9
Difficulty_AIVisual_Mult_Normal=1.0
Difficulty_AIVisual_Mult_Hard=1.15
Difficulty_AIVisual_Mult_Expert=1.2

AI auditory acuity multiplier - how well AIs can hear
Difficulty_AIAudio_Mult_Easy=0.8
Difficulty_AIAudio_Mult_Normal=1.0
Difficulty_AIAudio_Mult_Hard=1.03
Difficulty_AIAudio_Mult_Expert=1.13

AI tactile acuity multiplier - how well AIs can feel
Difficulty_AITactile_Mult_Easy=1.0
Difficulty_AITactile_Mult_Normal=1.0
Difficulty_AITactile_Mult_Hard=1.0
Difficulty_AITactile_Mult_Expert=1.0

AI hitpoints multiplier - all AI starting hitpoints will be scaled by this
Difficulty_AIHitPoints_Mult_Easy=0.75
Difficulty_AIHitPoints_Mult_Normal=1.0
Difficulty_AIHitPoints_Mult_Hard=1.2
Difficulty_AIHitPoints_Mult_Expert=1.3

AI combat speed multiplier - all AI combat animations are scaled by this
Difficulty_AICombatSpeed_Mult_Easy=1.0
Difficulty_AICombatSpeed_Mult_Normal=1.0
Difficulty_AICombatSpeed_Mult_Hard=1.1
Difficulty_AICombatSpeed_Mult_Expert=1.2

AI-to-player damage multiplier - all damage applied to the player by the AI is scaled by this
Difficulty_AIToPlayerDamage_Mult_Easy=0.5
Difficulty_AIToPlayerDamage_Mult_Normal=1.0
Difficulty_AIToPlayerDamage_Mult_Hard=1.25
Difficulty_AIToPlayerDamage_Mult_Expert=1.5

The higher the number, the better they see, the harder they hit, faster they are, etc. Edit the numbers to your liking, start a new game and enjoy.