There is a way to do that using what we call junction points/symbolic links. But if your C drive is that tight on space, honestly it's much more straightforward (and easier to manage) if you just use Windows to shift your entire Documents library to another drive. That is, assuming you have another internal drive to use; an external for this purpose is probably not going to work out well.
The game doesn't really care where your TS3 user game folder is physically, as long as it can find it through the system defined Documents directory.
Here are some
illustrated instructions on how to do that. Forgive the "dummies" website name, but they do offer some very good tutorials over there like this one. It's for Win 10, but the instructions for Win 8.1 shouldn't really be very different.
Edit: Okay, just in case
here are the Win 8.1 specific illustrated instructions.
