Post edited to add link and "caveat":
Second vote here for GOG.com
Sid Meier’s Civilization III Complete provides strategy fans with the classic Civilizatio
www.gog.com
As with Steam, you'd need to set up an account with them, and give them an email address, but the only 'spam' I get from them is notifications about sales of items on my wishlist, which I agreed to allow. The company's based in Poland, so is subject to EU legislative and consumer-protection standards.
All due respect to WJ, but the price he quoted is a sale price. Gog's standard price is the same as Steam's.
But (as of this edit, 12:39 CET, November 28th 2022) it is on sale right now, at €1.29 (or your LocalCurrencyEquivalent)!
The advantage of Gog over Steam is that the game can be installed and will run independent of Gog's client software. Gog is also compatible with
@Flintlock's patches, and nearly all Civ3 mods made between about 2004 and 2018, out of the box.
The "advantage" of Steam is the ability to join online multiplayer games. The downside is that Steam users will need to tweak most mods'
labels.txt file, to make the mod compatible with the multiplayer-capable executable that Steam provides.