OneAlex is a "supermod" all-in-one- You donīt need steam SH3, or other version. Copy-paste the SH3 folder of OneAlex as it is, and you have the game. Run Hsie app and check you have enabled the 4GB patch. This is a must have condition, without 4GB patch the mod doesnīt work. You need a medium-power PC to run it, laptops with integrated GC and/or low memory or low CPU power cause problems...It is a heavy mod. Really, all the actual supermods for SH3 are heavy; for low end computers you can play SH3 with GWX in his original version (and it is not so easy).
Good luck and Happy New Year!
Fitzcarraldo