@ronjlund I take it your Mac runs Catalina? Sims 3 hasn't been updated to work in Catalina yet, and we don't have an ETA for the new version. Until then, there's no way for you to play the game in Catalina.
If you have at least temporary access to a Mac running Mojave or earlier, you could install Mojave on an external hard drive or USB and boot into that when you wanted to play. Or you could partition your hard drive and install Windows on one side; this is known as Bootcamping and is supported by macOS. You'd want at least 100 GB storage to dedicate to either project. Let me know if you have more questions about either one.
