I can't open Sims 3 and 4 save files

@NitroIndigo  The question is what folder these games are reading, and it's easy to find out.  Please start a new Sims 3 save, and when you save your progress, call it something distinctive that you can search for without turning up dozens of other entries.  Quit the game, open a File Explorer window, click This PC in the left panel, and search for that save name.  When you see the save, click on the folder, click "Open file location" at the top of the window, and you'll jump to its location.

Double-check that Sims 4 is saving to the same directory: create a new save, go back to the Main Menu, click Load Game, and hover over the folder icon below your other save into.  Let me know whether the file path is the same, at least until it branches into the Sims 3 and Sims 4 folders.

If you want, you can simply move your saves and other content to the folders the games are using.  If you want those folders to be somewhere else though, please post the file paths here, along with the location you'd prefer the games to use.