Ok, done. Maybe I haven't worked enough to solve that, but from the first time I worked with new Version I really got embarassed about the significant change. :(
Though, I am willing to learn :)
Problems are:
#1. The Track I want to record is muted during recording. I cannot hear the previous recording, though I need that kind of "overdubbing" or how would you call it.
In X2 the existing midi notes and my playing are played back both. In X3 I cannot hear the existing notes but only my realtime playing. Maybe there is an option I haven't seen, yet?
#2. After having recorded the new clip notes I want to combine both clips.
In X2 I just have to mark them and combine them with right click.
In X3 the notes of the old clip are greyed out at the place where new clip is. When I combine the two clips, old notes will be erased. If I drag the new clip on another track, the old one will be cut at that position. So I cannot combine them easily. At the moment I just found a way by making more steps than before.

