Lista de Posts da Mesma Categoria sem Plugin

 
<?php if ( is_single() ) { 
     $cat = the_category_ID(FALSE) ;
     $post = get_the_ID();
     $args = array( 
        'cat'=>$cat,
        'showposts' => 3, 
        'post__not_in' => array($post)
);
    ?>
    <h4>Relacionados</h4>
    <ul>
         <?php $recent = new WP_Query($args); while($recent->have_posts()) : $recent->the_post();?>    
             <li><strong><a href="<?php the_permalink() ?>" rel="bookmark"><?php the_title(); ?></a></strong></li>
         <?php endwhile; ?>
    </ul>
<?php } ?>