You won't be able to get the Mac version from Firaxis. I'm have Civ3 Complete on my Retina iMac running Sierra, latest version. I don't play it seriously, and really only use it to support the Game of the Month activities in these forums. It seems to run OK - both Play the World and Conquests versions.
Civilization 3 MapEditor 1.32.1 - Civ3 game editor. Download the latest versions of the best Mac apps at safe and trusted MacUpdate.
I bought it on CD when it first came out from Aspyr, but I doubt if the CD is even available now. If you are able to find a working, legal installer, you need to be careful about updaters - they typically depend on older Mac coding standards ('Carbon' if anyone else remembers?) that are no longer supported by recent Mac OS versions. I think I only still have my working versions because they migrated from my old systems into my current one. You have a rather better chance of getting it working if you can install a Windows virtual machine - Parallels, VMWare or maybe VirtualBox, and then get a copy of the PC version on CD or Steam.