You wouldn't necessarily need to use that method. You can assign Ctrl-1 or Cmd-1 etc to them by simply creating 3 scripts, 'Go to Layout 1', Go to Layout 2' etc, ticking them to show in the scripts menu (that is the default anyway) and moving them to the top of the list.
If all you are doing with the 3 scripts is making text etc larger each time, then you can make the scripts stay on one layout, and attach zoom levels to each shortcut, using the same idea.
You can try using an OnLayoutKeystroke trigger to perform an If Step that uses Get (TriggerModifierKeys) to detect that the alt key was down and Get ( TriggerKeystroke ) to detect whether 1, 2 or 3 was pressed.
You can use exit Script [false] to make FileMaker ignore this keystroke when alt-1, 2 or 3 is pressed and do nothing to pass the keystroke back to FileMaker.
Not all such key combinations trip this trigger so you'll need to test to make sure that it works.