Thank you, WebMachine, for your response and willingness to help.
I’ve managed to create my own override. However, the order is not correct. I wanted the posts to be in order by the category ID then by date in descending order. Meaning 5 (Wellness), 3 (Food), 2 (Fitness). The results seem rather scattered.
/* CUSTOM POST OVERRIDE TO DISPLAY POSTS FROM THREE SPECIFIC CATEGORIES, THREE POSTS EACH, SORTED IN DESCENDING ORDER BY DATE */
<?php query_posts( array ( 'category__in' => array(5,3,2), 'posts_per_page' => 3, 'orderby' => 'date', 'order' => 'DESC' ) ); ?>