Ok, try this (untested, so excuse any minor typos/syntax errors).
I have amended so that The Loop first checks that there is Posts to show. I’ve also removed the need for $count and the addition of ' ' by adding any matches to an array, and then outputting them using join().
Hopefully I’ve not gone down the incorrect route and it will be suitable for your needs.
<?php
/**
* The Template for displaying all single posts.
*
* @package WordPress
* @subpackage WDM
* @since WDM 1.0
*/
get_header();
?>
<div id="primary" class="site-content">
<div id="content" class="clear" role="main">
<?php
if(have_posts()) : while(have_posts()) : the_post();
$excludedcats = array(3);
$categories = get_the_category();
$title_cats = array(); // Initialise the arry to avoid errors when checking it
foreach($categories as $category) :
/** Check to see if the category needs to be added to the class="" of the title */
if(!in_array($category->cat_ID, $excludedcats)) :
$title_cats[] = 'heading-'.$category->slug;
endif;
endforeach;
/** Output the title. We are checking that there were actually category headings here as well, so that class="" is properly formatted */
echo '<h1 class="category-heading'.
sprintf(__('%1$s'),
/** %1$s */ (!empty($title_cats)) ? ' '.join(' ', $title_cats) : ''
). // end sprintf()
'">'; // end <h1>
single_post_title();
echo '</h1>';
/** Output the content */
the_content();
?>
<div class="related-posts-div">
<?php //wp_related_posts()?>
</div>
<nav class="nav-single">
<h3 class="assistive-text"><?php _e( 'Post navigation', 'wdm' ); ?></h3>
<span class="nav-previous"><?php previous_post_link( '%link', '<span class="meta-nav">' . _x( '←', 'Previous post link', 'wdm' ) . '</span> %title' ); ?></span>
<span class="nav-next"><?php next_post_link( '%link', '%title <span class="meta-nav">' . _x( '→', 'Next post link', 'wdm' ) . '</span>' ); ?></span>
</nav><!-- .nav-single -->
<?php comments_template( '', true ); ?>
<?php endwhile; ?>
<?php endif; ?>
</div><!-- #content -->
</div><!-- #primary -->
<?php get_sidebar(); ?>
<?php get_footer(); ?>