Hi @winfriedkoch0,
With more than two variables things do get a little more complicated. One thing you can do is to use more than one variable in the "Group X" and "Group Y" drop zones by dropping another variable in the nesting zone on the left or right (see gif below) to retain fixed width sizing. Another option (which might be useful if you have a large number of variables and don't want the hierarchical heading) would be to make a new variable that's a concatenation of several other categorical variables and then use that composite variable as the Group X.
Hope this helps!
@julian