Re: The Sims 3

@isthatjessie  Your Mac's supported scaled resolutions are 1680x1050, 1440x900, and 1024x640.  While you should in theory be able to play in fullscreen at 1280x800, since it has the same aspect ratio, it's possible that this has odd effects in fullscreen mode, so please try using one of the supported resolutions instead.

What OS are you currently running?  If you haven't updated to Monterey, I wouldn't do that now—people on newer Macs have said they only have a couple in-game resolutions available.  I don't know whether your computer would be affected as well, but since it's a pain to roll back an OS update, better to avoid it.  But if you're on an older OS and don't have a reason NOT to update, installing the next-newer one may help.

Other than that, the only suggestion I have is to use windowed mode and expand the screen by dragging the edges until you get a size that works for you.  This kind of flickering is generally a system issue, or at least an issue of how the system renders the graphics, and not necessarily an in-game issue that can be addressed from that end.