Dealing with listing multiple categories
I don't think I quite follow you, so forgive me if I'm way off. I have created (what I think) is something similar to what you are looking for. This is how I did it - maybe it will help...

Let's say I have the following url:

http://www.mystore.com/toys/my-awesome-toy


twig

{% set cat = craft.categories.slug(craft.request.getSegment(2)).first() %} {# Toys #}

{% for toys in craft.categories.group('toys').descendantOf(cat).level(2) %}
    {% if toys.getDescendants().total() != 0 %}
        {% for subToys in craft.categories.group('toys').descendantOf(toys.id).level(3) %}
            ...
        {% endfor %}
    {% endif %}
{% endfor %}