Legal Civ3 assets.
Civ3 on Steam is a community add-on for OpenCiv3. Steam Civ3 Complete. Use the PlayBound launcher to drop the archive into the correct OpenCiv3 content folder, then enable the mod from the game's mod or content menu. Mods are versioned separately from the base game — match the recommended engine or client release before joining multiplayer, and keep a clean backup of your settings if you experiment with several packs at once. PlayBound only lists projects that publish redistributable archives or clear external download pages. Always read the upstream README for credits, required assets, and known incompatibilities.
Download page
This package isn't one-click yet. Open the official download page in your browser, then place files in the game's mods folder.
Install OpenCiv3 first if you have not already — Civ3 on Steam is an add-on and needs the base game.
The quickest route is the PlayBound Launcher: open OpenCiv3, find Civ3 on Steam in its mods list and press Install. The launcher downloads the official release and places it in the right folder for you.
Civ3 on Steam is distributed through https://store.steampowered.com/app/3910/Sid_Meiers_Civilization_III_Complete/. Follow the instructions there — some add-ons install through the base game's own content browser rather than as a file.
Start OpenCiv3 and enable Civ3 on Steam from the in-game mods or add-ons menu. The download is roughly 40 MB.
Yes. Civ3 on Steam is released under Community / External hub and costs nothing. Neither it nor OpenCiv3 has any paid content.
OpenCiv3, which is itself free. Civ3 on Steam is an add-on and will not run standalone.
The PlayBound Launcher installs it in one click from OpenCiv3's mods list. You can also install it by hand — full steps are on this page.
Roughly 40 MB, on top of the base OpenCiv3 installation.