single.php

<?php
/**
 * Single
 *
 * @package      EAStarter
 * @author       Bill Erickson
 * @since        1.0.0
 * @license      GPL-2.0+
**/

/**
 * Related Posts 
 *
 */
function ea_single_related_posts() {
  $loop = new WP_Query( array(
    'posts_per_page'  => 3,
    'post__not_in'    => array( get_the_ID() ),
    'category_name'   => ea_first_term( 'category', 'slug' )
  ) );
  
  if( $loop->have_posts() ): 
    echo '<section class="related-posts">';
    echo '<header><h3>Related Posts</h3></header>';
    while( $loop->have_posts() ): $loop->the_post();
      get_template_part( 'partials/archive', 'related' );
    endwhile;
    echo '</section>';
  endif;
  wp_reset_postdata();
}
add_action( 'tha_entry_after', 'ea_single_related_posts', 5 );

// Build the page
require get_template_directory() . '/index.php';