Thank you Charleski
It works - but it gave me a new problem.
I tried it like you said, and the TOC really looks great, exactly like I wanted it to ...
... but in the flow text, the roman number is not centered anymore. Of course, that is just logic, because the hidden part is also inside the h3 tag.
So I tried to center just the roman number with a div tag, and this works, too. Now I am happy
