@mangogirl10 Thanks for the deviceconfig. I'm not sure what changed either, but if you're up for a bit of experimenting, you can try playing with the graphics settings.
First, try deleting options.ini, in Documents > Electronic Arts > The Sims 3. The game will spawn a new copy the next time it loads, and the in-game settings will revert to their defaults. Don't change any settings initially; just see how things look.
If that doesn't make a difference, try playing in windowed mode. Hit cmd-enter at the Main Menu or in-game to switch.
If the flashing continues, please go to the Main Menu, reduce all the graphics settings to the minimum, quit, reload, and test. If the game looks better, you can try increasing one graphics setting at a time to see which one(s) cause more flashing. Each time you do so, please change the setting at the Main Menu, not while you have a save loaded, and quit to desktop and reload rather than immediately loading a save.
Please let me know which settings, if any, make a difference. If you still get the flashing even with everything on minimum and the game in windowed mode, try playing in a clean user data folder. Move your entire Sims 3 folder out of Documents > Electronic Arts and onto your desktop, and when you open the launcher, a clean folder will spawn with no content. Don't add anything to the folder yet; just see whether the game continues flashing. The new folder will mean a new options.ini as well, so you'll need to repeat the above process.
By the way, I hope you're restarting your computer often, but if not, now's a great time to do it. A real reboot, not closing and opening the laptop lid but actually telling the computer to restart, can fix all sorts of issues.