Help displaying related categories

Replace wp_get_post_tags($post->ID);
with

wp_get_post_terms($post->ID);

All parameter and details can be found here

and change the $args parameter

$args = array(
    'category__in'        => array($simlar_tag),
    'post__not_in'        => array($post->ID),
    'posts_per_page'      => 3,
    'meta_query'          => array(array('key' => '_thumbnail_id', 'compare' => 'EXISTS')),
    'ignore_sticky_posts' => 1,
  );

Hope it works!