By default the output of a WordPress query is to show a specific number of or all posts that belong to all the categories (or custom taxonomies). What if you want to output say only a maximum of 3 portfolio CPT items from each portfolio_category?
For this one approach is to:
get all the terms of the taxonomy in question
loop through them and for each term, get the IDs of recent 3 items into an array
flatten the array
run a WP_Query looping these IDs and output the items