How to downgrade The Sims 3?

starbeebee​  What previous version are you trying to run?  Sims 3 wasn't stable even in the beginning, so if you're looking for some perfect version, it doesn't exist.  And even if it did, the only way to run it would be to install the game via disc and use the old incremental patches to update to exactly that version.  There's no way to install anything but patch 1.69 (Windows) or 1.70 (macOS) through the EA App, or version 1.67 (Windows only) through Steam.

Mods have been necessary since the beginning, although the particular issues that made the game difficult to run did change over the years.  If you have a specific issue you'd like help addressing, let me know.

A lot of us use LazyDuchess' mods, which fix so much of the issues with The Sims 3. I want to be able to enjoy the versions before EA made the patch that broke those mods. 

starbeebee​  Are you talking about the patch that was released in January 2025?  I'm not aware that the update broke LD's mods, although my gaming computer has a Steam install and I only have an EA App/1.69 install for testing.  Still, I haven't seen any reports that that particular update broke anything other than core mods, and those were quickly updated by any modders still active.

But if you're certain the problem is the update, and not something on your system (I have seen reports that the Smooth Patch broke last fall, due to a Windows update), you can always buy the game on Steam.  It's still on patch 1.67.  You'd need to rebuy all your expansions and stuff packs too though.  Store content would transfer as long as you manually registered the base game code Steam provides on the Store site.  Don't link your EA and Steam accounts, as this seems to auto-consume codes without properly registering them, and therefore the Store content often doesn't work.