C-Drive decreased overnight from 18Gb free to 3 GB free space.

Your C: drive lost 15 GB overnight despite Storage Sense, so identify the category using space before deleting more. Open Settings > System > Storage > Show more categories, and record Installed apps, Temporary files, and System & reserved. Open Cleanup recommendations and review Temporary files, Large or unused files, Files synced to the cloud, and Unused apps. If Previous Windows installation appears, remove it only if rollback is unnecessary; this cannot be undone. Check Windows Update history for an update matching the space loss, because updates can temporarily need extra space. Move personal files to external storage or make eligible OneDrive files online-only. Do not manually delete WinSxS or other Windows folders. If Storage still does not explain roughly 15 GB, restart, run a Windows Security scan, and report the category totals and whether free space changes again. That evidence is needed before recommending removal of a system file.