yes, that is how you said you wanted it to work:
It only must be applied to one product, if another product or multiple quantities of a product from that category is added to cart it will be charged as normal. 
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
AND
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.
