<div class="single-post__related">
    <?php
      $categories = get_the_terms(get_the_id(), 'category');

      $cat_ids = array();
      foreach( $categories as $category ){
        $cat_ids[] = $category->term_id;
      }

      $related = new WP_Query([
        'post_type'       => 'post',
        'posts_per_page'  => 3,
        'category__in'    => $cat_ids,
        'post__not_in'    => array( $post->ID )
      ]);

      while($related->have_posts()) : $related->the_post();
     ?>

      <div class="single-post__related-item">
        <a href="<?php the_permalink(); ?>">
          <?php the_post_thumbnail(); ?>
          <h6><?php the_title(); ?></h6>
        </a>
        <span><?php the_time('F d, Y'); ?></span>
      </div>
    <?php endwhile; wp_reset_postdata(); ?>
  </div>