Both are in their default locations. EA App in Applications, Macintosh HD > Applications. The Sims 3 is in Macintosh HD > Applications > EA Games. I know that before, with Origin, The Sims 3 was being installed under Users > My Username > Applications, but now the default location has been changed for EA App.
@Cerulean_Bead A couple of players with a similar issue with Sims 4 had to add the game to Full Disk Access. So please try that too: open System Settings > Security & Privacy, click the lock to make changes and enter your password, click Full Disk Access on the left, and add Sims 3 on the right.
