What I have done:
I pinned the icon of k3b to the taskbar to autostart k3b.
Now I have two instances of k3b (version 19.04), which start automatically on start of desktop KDE.
When I close the two instances of k3b and log out of the desktop, then k3b does not start automatically.
When the computer is started into Manjaro KDE, then 2x k3b is started.