Hi Rosa,
Sorry about the late response, I did not forgot your message.
I had to take a closer look if this is actually possible somehow – but I’m afraid not without major custom coding. This kind of logic would work only if the items were not in the main category as well. The parent “main” category only differs from the other sub-categories by it’s level, otherwise it is a category just like any other.
So once you select A + A1 with “OR” logic, then items from A3 will also show up, if they are associated with the A category.
I was looking for a possible way to maybe alter the filters and remove the main category from the query arguments – that could actually solve this, but I don’t know if that is possible without any major changes.
I will get back to you a bit later, will let you know.