It depends on which Settlers 3 edition You did:
Settler 3, Settler 3 Mission CD, Settler 3 Amazons or Settler 3 Gold
Settlers 3 History Edition
Start uPlay.
Switch to the overview of your games in uPlay.
There select Settlers 3 by clicking on .
Go to „Properties“ on the left.
There you will see a box called „Local Files“. Click on the button „Open folder“.
This will open an Explorer window with your Settlers 3 installation directory.