Hi,
I understand what you are saying. But unfortunately not possible. As you might aware, official Virtuemart Category Module doesn't support displaying child-categories beyond 2nd level (i don't know why though).. With the SportMart template, we didn't re-write the category module, we just "styled" the existing one, so the available php objects, limitations, flaws etc.. are all carried as well.
There is no simple way to tweak the module and have it display 3rd level. But if you are okay with purchasing "Premium Support"
www.flexiblewebdesign.com/premium-support please let me know, I can spend time on your website to take care this modifications and have it display the 3rd or even 4th level categories. (Most likely i will have to create a brand new category module, instead of customizing the official vm category module though)
Regards