There is a copy on SubSim here, but I was unable to find it... so try this one
NTCore 4Gig Patch, which you would put in your game folder, extract, then run and choose the SH3.exe file, and you should then get a messagebox "Executable successfully patched [OK]". If you don't get that message, then the SH3.exe is write-protected, which in a Steam install is not the norm. If it is though, right-click on the SH3.exe file, and on the resultant context menu, choose "Properties" near the bottom, and on the first "General" page that opens near the bottom is the "Attributes" section, which might have the "Read-only" ticked. If so, clear the tick and OK out, then try the 4Gig Patch again.
A few points about a Steam install:
1. You should not have the game installed in a "C:\Program Files" folder. See Moving a Steam Installation and Games for more on how to move it. Expect any move to take a while, no matter which way you do it. If you have a valid 2nd hard drive, you can create another Steam Library folder on that, and put SH3 there.
2. The Steam version of the game does not like widescreen mods. You will have to experiment and probably have to get more help. I cannot remember how I got my Steam version to work with one, if I ever did. I no longer use my Steam SH3 game, but use a disk version now, and I no longer have to read the tea leaves and burn incense. Perhaps someone else will comment on this aspect.
3. Windows 10 will attempt to alter your video settings to make the SH3 game display "pretty and smooth", and in the process will mess it up. Be sure and experiment with the "Compatibility" tab of the SH3 "Properties" page to get things to run correctly.
Any more questions, be sure and holler!