Ahoy,
I, for 1, heartily endorse getting it from Steam, as a source for SH3.
Main point being... it is dirt cheap there & every once in a while, they do a 3 in 1 deal on SH set (which includes in it SH3, SH4 (v1.4 without Uboat Missions & v1.5, which is SH4+Uboat Missions aka SH4 Gold Edition the Ubi designation for it from their end of things...) & SH5) which is perfect, if you want to mod all of those.
On the + side of all this, there is a tool which will allow you to divorce SH3 from Steam, called SFSF. Can locate that by doing a google search for it by entering into the search bar ''[SFSF-MODULE]'' (minus the quotation marks).
This tool, is fairly simple to use, point it at where the SH3 is installed, designate where you want it to go, making sure that divorced SH3 is NOT installed (or sent)in either:
C:\Program files
or
C:\Program files (x86)
I recommend creating the following:
C:\Games\SH3\
& then send it to this folder... after that, make a copy of it (unmodded yet, of course) & then save that on a flash drive or if you have a mega account or some other similar site like it or even both... & save it as your gold standard copy to use later if you wish to do more than 1 modded SH3 flavor...
If you do want to do more than 1 modded out version of SH3, I can NOT stress the importance enough of, ensuring that you make sure of including in the folder along with SH3, MultiSH3. This tool, sets up a completely separate save game folder, well away from the main 1 that SH3 sets up the very 1st time you run it, so that the files complete their initial important set up.
As was pointed out, if you need to access it again due to any loss of the gold standard backup, you can always go back to Steam & redownload it.
Hope this info helps...
M. M.