@theagyas - Hi there. I'm afraid that your Mac is one with a processor/graphics chip combo that is among the most stubborn to get TS3 for Mac to recognize and get working. Those who can tend to follow the sgr file editing advice given by Bluebellflora both here and on her comprehensive blog that contains what I would say would be all everyone knows about TS3 and Macs. These are good places to start, but also have a look around her blog for further info and tips on playing the game, should you be able to get it running.
https://bluebellflora.com/unknown-error-message-sims-3-base-game-only
https://bluebellflora.com/fix-for-incompatible-graphics-for-the-sims-3-on-mac
There is also the option of Bootcamping Windows onto your Mac and playing the Windows version of TS3 instead. That's how I've been playing on my iMac for years. But that requires enough hard drive space to devote to both the MacOS and Windows partitions and the patience to manage two operating systems, plus you will still be held back on performance by not having an Nvidia or AMD dedicated graphics card on your system.
Some with the combo you have, though, will never be able to run the Mac version of the game. Your other recourse in that case would be to consider asking EA for a refund. Note that EA Customer Support doesn't live here, we are your fellow players on a community message board, so we're not the ones making the decision to continue selling TS3 for Mac or not. :/
Also note that if you intend to stay current with OS X, even if you are able to get the game running now, from all indications the next version of the OS beyond High Sierra that will come out in the fall (MacOS 10.14, it doesn't have a name yet) will drop all support for 32-bit applications. Therefore, again if staying current, that will be the end of TS3 for Mac entirely unless EA issues some kind of 64-bit fix for it which I'm afraid doesn't sound very likely, given the history here and how far out of development TS3 now is.
