@indycutie Please delete everything in Downloads with Origin in its name. Then use this guide to uninstalling "the hard way" to remove any leftover Origin-related files:
https://help.ea.com/en/help/origin/origin/manually-uninstalling-origin/
Restart your computer, and don't open the EA App. (Make sure it's not running in the Task Manager as well.) Download a fresh copy of Origin and run the installer as an admin: right-click on the download and select "Run as administrator." Restart again when it's installed, and again make sure the EA App isn't running. Then try to launch Sims 3 from your Origin game library.
If Origin doesn't think Sims 3 is installed, right-click on the game icon in your library and select Locate Game, then choose the location where the game is installed. It might help to run a repair afterwards (in the same right-click menu). Then launch the game from your Origin library and let me know what happens.