relatedposts.php

<?php
/* 
A WP_Query loop to get posts previous to the current post based on category
Most related posts plugins start from the newest posts but then related links are constantly shifting
This doesn't seem good for either context or internal linking SEO

However, this process uses a very large range() array and used ~.67MB of memory to look at my 3,000 post blog
*/
?>

<?php

// while still in the loop on a single post:

$firstpost = get_the_ID();
$range = range($firstpost, 1);

// This might be a suuuper large array, but let's continue: 

// Put all the categories in the current post into an array: 
$cats = array();
foreach(get_the_category() as $category) {
  $cats[] = $category->cat_ID;
}

// Set up the new query:
$args = array(
        'offset' => 1, // don't use the current post in the related list
      	'posts_per_page' => 3,
        'category__in' => $cats,
        'post__in' => $range
      	);

$the_query = new WP_Query( $args );

if( $the_query->have_posts()):
  while( $the_query->have_posts()) : $the_query->the_post(); 
// run loop code
?>

<a href="<?php the_permalink(); ?>"><?php the_title('<h3>', '</h3>'); ?></a>  
<!-- etc -->

<?php
  endwhile;
endif;
wp_reset_postdata();