Need help about Related Posts in current theme !

Hi everybody,
I'm using Wordpress last version is 3.9.1.
And my theme using Relate Posts Thumbnails without plugin. But it only show when same tag (ex: 2 posts have same 1 tag). Please help me edit the function to show Related Post Thumbnails via Category.

Code in function.php below:
function get_related_posts($post_id, $tags = array()) {
$query = new WP_Query();

if($tags) {
foreach($tags as $tag) {
$tagsA[] = $tag->term_id;
}
}

$args = wp_parse_args($args, array(
'showposts' => 3,
'post__not_in' => array($post_id),
'tag__in' => $tagsA,
'ignore_sticky_posts' => 1,
));

$query = new WP_Query($args);

return $query;
}

Code in single.php below:
<div class="post-box-wrapper">

<div class="post-box">

<h5>Related Posts</h5>

<?php while($related->have_posts()): $related->the_post(); ?>
<?php if($count == 4): $count = 1; endif; if($count == 3): $class = 'last'; else: $class = ''; endif; ?>
<?php if(has_post_thumbnail()): ?>
<?php $image = wp_get_attachment_image_src(get_post_thumbnail_id( $post->ID), 'related-posts-image'); ?>
<div class="related-item <?php echo $class; ?>">
<a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>"><img src="<?php echo $image[0]; ?>" alt="<?php the_title(); ?>" /></a>
</div>
<?php endif; ?>
<?php $count++; endwhile; ?>

</div>

</div>

Pro wordpress code please help me edit.
Thanks so much.