You actually doing mistake. You have to pass 3 parameters as array not like this.
Use below code
<?php //if is single event, guide, firm news
if ( is_singular( array('events','guide','firm-news') ) ) : ?>
<h5 class="event-subpage-header-title"><?php echo $title; ?></h5>
<?php
else :
?>
<h1><?php echo $title; ?></h1>
<?php
endif;
?>