Assuming you are refering to the code of this answer
Taking the query from that answer
$args = array(
'category_name' => 'your-category-slug',
'posts_per_page' => 3,
);
$wpse_235685_query = new WP_Query( $args );
you would then, to reverse the order of your results, need to modify the query $posts
object, like so
$temp_posts = $wpse_235685_query->posts; // store the posts to work with
$wpse_235685_query->posts = array(); // empty the $posts object
$wpse_235685_query->posts = array_reverse($temp_post); // set back the object to use new reverse order
of course this could be simplified by getting rid of the temporary variable like so
$wpse_235685_query->posts = array_reverse( $wpse_235685_query->posts );
This code should go between your if( $wpse_235685_query->have_posts() ) :
and your while( $wpse_235685_query->have_posts() ) :
so it doesn’t run when the query returns an empty set.