@CourtnieCullen There are several steps necessary to make TS3 run on that computer. It should be noted that you have a processor/graphics card combination that's incompatible with the game, so there's no guarantee the process will work. But if you want to try, there are several handy guides to help you. Also, if you're already tried to install and it hasn't worked, or you previously played on this computer, it's best to do a clean uninstall before proceeding. The uninstaller that comes with the game misses some files that can interfere with a later install.
https://bluebellflora.com/how-to-do-a-clean-uninstall-and-reinstall-on-a-mac/
On this computer, you need to install through Origin, as High Sierra won't authenticate discs. It's best to start with only the base game at this point. Then run the Super Patch (digital download version) rather than patching through the launcher.
http://forums.thesims.com/en_US/discussion/549552/official-ea-download-links-for-the-sims-3-cumulative-manual-update#latest
At this point, you can try to launch the game. You'll probably get an unknown error message, due to the processor/card combination. The solution to this is to edit your .sgr files. BluebellFlora has written out some very helpful instructions for doing so. Just figure out your mac's model number and then follow the corresponding link. Be sure to make copies of your .sgr files before you start, in case something goes wrong.
https://bluebellflora.com/fix-for-incompatible-graphics-for-the-sims-3-on-mac/
Now, you can try to launch the base game again. If it works, you can start installing packs. There are four packs—Ambitions, Late Night, Fast Lane, and Outdoor Living—that will throw another unknown error and need extra interventions. But these steps are much simpler than the one above. The first thing to do is to create an alias for any pack that does not already have one in your Applications folder, alongside the alias for the base game.
https://bluebellflora.com/sims-3-packs-not-showing-as-installed/
The second step is only necessary if at least one of the affected packs is NOT the newest pack by release date. (So if you only had, for example, the base game, High End Loft Stuff, and Late Night, you could skip this step.) Certain files don't get copied from the affected packs into their proper destination, which is the folder for the latest pack to be released. If you had all packs, that would be Into the Future. You can copy the files manually by following the instructions in the video embedded in the post below. This thread also discusses all the unknown error issues that affect mac users.
https://answers.ea.com/t5/The-Sims-Mac/CURRENT-ISSUE-Sims-3-Unknown-Error-in-El-Capitan-Sierra-High/m-p/5725514#M5926
If all this doesn't work, please post back here listing all of your mac's specs, as well as which troubleshooting steps you've tried and any error messages you've received.
mac specs: https://answers.ea.com/t5/The-Sims-Mac/READ-FIRST-Sims-3-Mac-Help-Guides-and-Troubleshooting/m-p/4756624#M4
