You might be thinking, "But I want a standalone installer!" I get it. But here is why Steam is actually the best way to get the normal experience:
For those who want the most "normal" experience possible, here is the exact process using the GOG offline installer:
That is it. No BIOS updates. No command-line edits. No cracked DLLs.
If you truly want a normal, DRM-free, offline installer (the kind you can back up to a USB drive), then you should buy it on GOG (Good Old Games).
GOG Link: https://www.gog.com/en/game/dungeon_siege_iii
GOG is the hero of game preservation. The version there is clean, patched, and has no copy protection. You buy it once, you own it forever.
Here are the only three legitimate sources for a direct download of Dungeon Siege III. These are as "normal" as it gets.