Thank you @hilbrand - that guideline is very valuable!
I decided not to change the existing configuration but to simply reinstall Eclipse setting the OH3 branch. I understood that you did the same. I am now able to startup app.bndrun, however it does not load my binding. I added my binding into the Run Requirements of app.bndrun and I added my binding to org.openhab.demo.app/pom.xml. Any other place where I have to add or register my binding to have it loaded? I even added my binding in git/openhab-addons/pom.xml, but without any effect.
There should be no other requirements. In the documentation about eclipse ide with openHAB this is described. Eclipse IDE | openHAB Also check the animation, which shows how to launch the debug environment with a binding.