yes, that is how you said you wanted it to work:
perhaps I missed something? i read that as meaning that the discount only applies if one item in the category is in the cart, if a second item in the category is in the cart, the discount does not apply, if that is not the case, change the trigger to
Number of Items with a specific column value-
if the total number of items where 'category' has a value of '3' is '>=' a value of '1'
if the discount applies when multiple items are in the cart, then the discountAmount column cannot be used, you will need to hard code the discount amount, it cannot be dynamic.