I installed Sims 3 a few years ago and I didn't play it for a year, so Origin needed to update. I also needed to log in to my Origin account, but I don't know my email, password or ID anymore, so I made a new one, but that account doesn't have Sims 3 in the launcher. Since it was already installed earlier, I can add this manually, but I don't know which file I should pick to launch the game.
I dont have Sims 3 installed at the moment, but it works the same way as other Origin games. The file you want is the .exe file.
The game folder is located under (as default): C:\Program Files (x86)\Origin Games
Once you are in the Sims 3 game folder look for an .exe file called Sims3 (or similar). That's the file that launches the game.
Hope this answer was of any use. If you have any questions leave a comment.