@klaus, if checkboxes 4, 5 are not set as 'required' (i.e. the backend component is not really necessary), then a dedicated JS script could do the trick and also take care of the values upon clicking Save. You can ask me for that script. Also it may be wiser to exclude Conditionals from the problem itself, show all 3 checkboxes and upon clicking one of them - 'disable' the ones that need not be clicked. Making Conditionals work 'backwards' as you asked is not possible.
If you could cook something up with JS that would be most helpful, thank you very much! As you said yourself showing all three possibilities and than 'disable' the ones when another is clicked is exactly what I would like to achieve.
However the bunch of checkboxes are only a part of a larger survey and I would like to make use of the multi step form approach (viewtopic.php?f=2&t=7134&start=10#p9596). Would that still work?
But it is still intriguing if one could show/hide other parts of code as well (like a hint or an error message). Any advice on that?