How to: display sub-items ONLY if parent-item is selected

Hi everyone,

So here’s another question about listing menu items with more than 1 depth. I’ve created a menu for a webstore with 3 ‘layers’. Currently it looks like this:

Item 1
– child a
– sub-child I
– sub-child II
– child b
Item 2
– child a
– child b
Item 3
– child a
– sub-child I
– sub-child II
– child b

My goal: I’d like to display ‘Sub-children’ (3rd layer) ONLY if their parent (2nd layer) is selected.

I’ve searched through a lot of topics and tried a lot of different solutions but i just can’t get it fixed. Is there someone who had to deal with this problem recently? I appriciate everyone’s input.

Thanks in advance!