For NixOS it's just a matter of adding the following to your system configuration, I'm not sure about other distros.

  services.xserver = {
    enable = true;   
    desktopManager = {
      default = "xfce";
      xterm.enable = false;
      xfce = {
        enable = true;
        noDesktop = true;
        enableXfwm = false;
      };
    };
    windowManager.i3.enable = true;
    displayManager.defaultSession = "xfce+i3";
  };