FTry uninstalling SMM2 from the Windows Settings (Add or remove programs) or Windows Control Panel.
If uninstalling SMM2 from the settings or control panel still doesn't work, install a version of SMM2 (https://github.com/satisfactorymodding/SatisfactoryModManager/releases/tag/v2.9.3) such that there is something valid to uninstall, then try installing SMM3 again
If uninstalling SMM2 from the settings or control panel still doesn't work, install a version of SMM2 (https://github.com/satisfactorymodding/SatisfactoryModManager/releases/tag/v2.9.3) such that there is something valid to uninstall, then try installing SMM3 again