How to write a segment that is not two other segments
I personally try to avoid inlining too many conditionals because I feel like it leaves room for logical loopholes. I also think using variables for segments keeps things easier to read.

{% set seg2 = craft.request.getSegment(2) %}
{% set seg3 = craft.request.getSegment(3) %}


So maybe something like this? The keyword in is Twig's way of checking in_array()

{% set badItems = ['itemA', 'itemB'] %}

{% if seg2 == 'Subcategory' %}
    // do top priority thing
{% elseif seg3 != '' and seg3 not in badItems %}
    // do secondary thing
{% else %}
    // do overall default thing
{% endif %}




However, if you know the finite list of acceptable items beforehand, I think this is more concise.  Since segment 3 could be blank if it's not there, maybe try checking if it is something instead of if it's not something.

{% set goodItems = ['itemC', 'itemD'] %}

{% if seg2 == 'Subcategory' %}
    // do top priority thing
{% elseif seg3 in goodItems %}
    // do secondary thing
{% else %}
    // do overall default thing
{% endif %}