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. 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.