i3 is not available for Mac but there are some alternatives that runs on macOS with similar functionality. The best Mac alternative is Xmonad, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 25 alternatives to i3 and six of them are available for Mac so hopefully you can find a suitable replacement. Other interesting Mac alternatives to i3 are yabai (Free, Open Source), Amethyst (Free, Open Source), spectrwm (Free, Open Source) and Phoenix (Free, Open Source).
xmonad is a dynamically tiling X11 window manager that is written and configured in Haskell. In a normal WM, you spend half your time aligning and searching for windows. xmonad makes work easier, by automating this.
Amethyst is a tiling window manager for Mac similar to the xmonad tiling window manager popular on Linux. Amethyst is written in Objective-C and has configurable shortcuts, multi-monitor support, multiple layouts, and the option to float certain applications.
spectrwm is a small dynamic tiling window manager for X11. It tries to stay out of the way so that valuable screen real estate can be used for much more important stuff. It has sane defaults and does not require one to learn a language to do any configuration.