You can download and run the work in progress. There are dozens of issues open and being worked. The new UI has been posted about on openHAB’s twitter feed. There is a thread to start working on a getting started guide with lots of screen shots.
There is a lot of “official” information out about openHAB 3. For this threads particular case there is even an entire page dedicated to creating the model: Getting Started with OH3: rewriting the tutorial - 6. Model your home with Items & link Channels