>the easiest and most affordable
Make up your mind, you can only have one or the other.
Ok, I'm not actually sure that's true, and I haven't tried Wine in a decade or two. I think WineX would do it, though. Civ3 needs DirectX 10 I think.
I have no idea how to get Steam to install Windows software on a Mac, though, or if you can. Civ3 seems to be on sale at GoG.com for $1.49 right now, usually priced at $5.99. With GoG you can download the install files and install yourself which I presume would be easy enough with WineX, but then again I haven't touched it in a decade or two, and it required a lot of tweaking way back then.