That's right... it will be a long script because, based on the selection, you need to use removeOption & addOption and adding after removing will add the choice at the bottom. So for every selection change, you would be clearing all options and adding the required in sequence you need... 

Why not make Category and Subcategory both dependent on Incident Type?

Or may be create a custom lookup table with the mapping of Incident Type/Category/Subcategory and use it to show hide choices.