Question: how to fix unknown error occurred sims 3?
Answer: Go into the application, "Disk Utility"
You're gonna see a thing-a-ma-bob that says "Recovery HD"
Click it.
Now click "Restore"
Now "Recovery HD" should be in bold print.
Now uninstall everything Sims 3 (doing a back-up before this is pretty imperative!)
Start installing again.