i replied to in your other* thread 🙂 if a game is installed via orign or disks then they can be uninstalled via the control panel , steam has it's own/system way of installing games
http://answers.ea.com/t5/Answer-HQ/Uninstall-Expansion-Pack-on-Steam/m-p/2821760
edit to add other * which i had omitted
edit edit
is your the sims 3 base game from a disk or origh ? as taht maybe what the issue is orign and disks work together, steam does not work with orign and or disks steam versions only work with steam you can get your code from steam and redeem it on your orign account then download the expansion pack from there
