Ok, so now is known the source.. BUT.. that still does not answer the important question.
Where did you install it on your hard drive...
Know that the default assignment by either Steam (or any other launcher shoved up your aft section to run a game you paid for..) defaults to the same location, as does a disk install...
C:\Program files
or
C:\Program files (x86)
every bloody time... unless you derail that & tell it somewhere else to drop the install at.
If you allowed the install of Steam, it's associated stuff, & that CD/DVD copy install of SH3 to get dumped into the default.. then therein lies part of your issues.
Before Win 10, most of the time, you were pretty safe to do so.. as far as I know of. With the advent of Win 7/8/8.1, Windows started the problems with what they tagged as 'UAC'. Meaning a file overlord, that does NOT like & is NOT amused with you tampering with anything under its iron fist.
That includes even simple editing of say.. basic files like GameplaySettings.cfg or main.cfg, found in the Data\Cfg folder in SH3.. & that's just simple file editing done there...
It gets worse, if you try & do add in simple mods.. much worse, if you go to do an add in of say.. a mega mod like.. oh.. GWX for example..
So, that is why it is important, to NOT do a 'default' install. Rather, creating your own folder, still on C: as that IS your main drive you will in all likelihood, will want your game/sim located.
Hope this info has helped...
M. M.
