Amiebriggs123 First, try to find the codes on your old PC, which should still be present if the game is still installed. Here's how to do it in Windows:
https://sims3.crinrict.com/en/2013/09/how-to-find-your-serial-number.html
If the codes were never registered, you can do that now.
If the codes were registered, take photos for your three discs showing at least the disc and case. Contact EA customer support, upload those photos, and have all of the codes on hand so that the support rep can look up the account that redeemed them. The rep may be able to help you figure out what email you used at the time, or if you have enough information about the account, even merge your current one into it.
If you do recover access to an old account and want to merge, keep in mind that Store content cannot be merged at all. So whichever account owns that content is the one that needs to survive, and the other account needs to be merged into it. You can always change the account details later (username, email address) if you want.
I would strongly suggest using the live chat option rather than email. If you don't see it, try this link:
https://help.ea.com/en/help-contact-us/?product=the-sims-3&topic=missing-game&category=manage-my-account&subcategory=find-missing-game
Make sure you're signed into an EA site (this one is fine) before clicking the link and that the location in the lower-right corner of the browser window is set to U.S. or U.K.
Alternately, try using the help site without being signed in. Some categories of issue allow you to proceed without an active login, and many of those offer the live chat option.
It's fine that Monte Vista doesn't show up in the EA App. The important question is whether it shows as owned on the Sims 3 Store site, which is where you'd need to download it anyway. It might be attached to your old account though.
