How to develop a cross-platform custom GUI for Android, iOS, Windows Phone?

Question

User interface

How to properly approach the development of a cross-platform application with an "unusual" interface.
1) In order to easily port it to ios, android, windows without changing the interface.
2) There was a lot of freedom in creating GUI elements.
Now I'm playing with openframeworks, there is already a rather large ofxUI library that can be heavily modified to suit your needs, you can also make your own GUI from scratch with due perseverance. But it seems to me that I shoot sparrows with a cannon, and everything can be done more elegantly.
There are also a lot of applications in the apple store with a serious custom interface (there are thousands of sequencers), I understand correctly that they are all written from scratch using core graphics?
ps
An example of applications with an unusual interface: pacemaker, auxy.
I would also be very grateful if someone advises something to read about the development of interfaces (the language is not important).

Answer 1

Perhaps, if the complexity of future applications is not very high - using Phone GAP
You make the whole UI on HTML5 + CSS, logic on JS.
Further, the application is wrapped in a webView and looks exactly the same on all mobile devices.
If there is a need to access the native functionality of the device (camera, microphone, compass, etc.) - phoneGap provides such opportunities, as if "connecting" additional features in JS.
There are many features of development in this way.
There are many disadvantages (I will not list)
There are two undoubted advantages:
1) Quick "move" to any platform
2) The ability to easily create any UI