Style Posts From Some Category Differently for Wordpress codex

<!-- Start the Loop. -->
 <?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>

 <!-- Test if the current post is in category 3. -->
 <!-- If it is, the div box is given the CSS class "post-cat-three". -->
 <!-- Otherwise, the div box is given the CSS class "post". -->

 <?php if ( in_category('3') ): ?>
           <div class="post-cat-three">
 <?php else: ?>
           <div class="post">
 <?php endif; ?>


 <!-- Display the Title as a link to the Post's permalink. -->

 <h2><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></h2>


 <!-- Display the date (November 16th, 2009 format) and a link to other posts by this posts author. -->

 <small><?php the_time('F jS, Y') ?> by <?php the_author_posts_link() ?></small>


 <!-- Display the Post's content in a div box. -->

 <div class="entry">
   <?php the_content(); ?>
 </div>


 <!-- Display a comma separated list of the Post's Categories. -->

 <p class="postmetadata">Posted in <?php the_category(', '); ?></p>
 </div> <!-- closes the first div box -->


 <!-- Stop The Loop (but note the "else:" - see next line). -->

 <?php endwhile; else: ?>


 <!-- The very first "if" tested to see if there were any Posts to -->
 <!-- display.  This "else" part tells what do if there weren't any. -->
 <p>Sorry, no posts matched your criteria.</p>


 <!-- REALLY stop The Loop. -->
 <?php endif; ?>