OK… I finally got it. This is what the correct query should have been, combining ACF and CPT arguments:
$relatedCat = get_field('service_related_industry');
$args = array (
'posts_per_page' => 3,
'post_type' => 'case_study',
'tax_query' => array(
array(
'taxonomy' => 'cs_industry',
'terms' => $relatedCat,
'field' => 'term_id',
),
),
'tag_id' => '"'.$relatedCat.'"',
'orderby' => 'DESC',
);