How can I easily run multiple sites with PHP, Apache and mySQL for free on mac?

All of these options have a GUI which is helpful. You could install Apache/MySQL/PHP with Homebrew or MacPorts, but you would be managing all of the settings manually. While this is the best way to get all of the options for free, this increases the difficulty curve.

As another option, I have Abyss Web Server running on my machine to act as a local dev server. The free version allows for 1 site though, there are more features with the paid version.

As with any of these options, you will have to pay for the extra features when using a GUI.

I looked into this years ago when Apple Server abandoned web services. I hope you find something that will work for you.