@SuperNTG Is OneDrive running on your computer? Even if you never use it, OneDrive can interfere with saving and with mods or other manually added user content.
If OneDrive isn't present, or it's unlinked (i.e. you're not signed in), what antivirus do you use? An AV could also interfere with user data.
@SuperNTG OneDrive is the more likely culprit here. The question is whether you want it gone permanently or just want to keep it from interfering with your Sims 3 data. I wrote this guide with Sims 4 in mind, but the options are the same for any folder in Documents:
https://answers.ea.com/t5/PC/CURRENT-ISSUE-Sims-4-saves-user-content-disappearing-PC/m-p/12475370
If you need more help with the process, let me know which option you'd like to use.
Separately, make sure you're adding the mods to the folder Sims 3 is actually reading. You can usually tell which one that is by the time stamp on DeviceConfig, which should always match the last time you launched the game. If it doesn't, that's not the right folder.
