A feature is planned which will allow multi-monitors on the server side to be treated as separate windows on the client side. In each of theses separate windows it will be possible to apply all the usual settings, like applying a different scaling factor, resizing the window to any size and placing the window in any desired location. It comes close to what you would like to achieve, in that n in your case, you could keep 2 monitor windows open whilst minimizing the third one.
Unfortunately, we cannot give a definitive ETA. Development priorities are currently on NoMachine Network (www.nomachine.com/network) and v9. Work on the feature you are interested in may start after that.