How can I emulate ctrl-Z (undo) in Android 4.1.2 on a Galaxy Note 3?
For anyone who comes across this via a search I found these options...


Polaris Office's word processing program implements undo that works via control-Z on my Galaxy Note 3 using a Bluetooth keyboard.
The JotterPad X text editor implements undo and redo that works fine via on-screen controls.

The JotterPad X app description on the Google Play store claims specifically...


  Compatible with physical and Bluetooth keyboard including shortcut keys (E.g. ctrl + Z for undo)


On my system, JotterPad X's undo didn't work with a Bluetooth keyboard, but maybe it will work on your system or with a different model keyboard.