The TS3 user game folder (not the program installation) that contains player added content, our game saves, game options file, etc. needs to be in the Windows user Documents folder. But the Documents folder in its entirety does not need to stay on C. The game doesn't care on which physical drive the Documents folder resides but it cannot (without a trick to be mentioned below) be located elsewhere.
How to redirect the location of the Windows user Documents folder in its entirety:
https://www.dummies.com/computers/operating-systems/windows-10/how-to-change-the-location-of-user-folders-in-windows-10
(This is for Win 10, for Win 7/8.1 it's very similar; sorry for the "dummies" link but they provide some really good illustrations over there)
It would be a really good idea to have a backup of that game folder made first just in case the redirection process loses something along the way. And also check for unwanted new game folders spawning in OneDrive if OD is not totally disabled and uninstalled.
If Documents really needs to stay on C for some other reason, then symlinks could be used to redirect just the TS3 portion of it to a different drive. These are pretty fussy to work with though and I would shy away from this strategy unless it's really necessary.
