deepbevel on "Best Way To Exclude Single Category In index.php of Theme"

try this

<?php query_posts('cat=-3'); ?>
<?php while (have_posts()) : the_post(); ?>

<div class="post">
<div id="post_title"><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent lenke til: <?php the_title_attribute(); ?>"><?php the_title(); ?></a></div>

<div class="date"><?php the_time(get_option('date_format')); ?></a> - Posted in: <?php the_category(' & '); ?></div>
<?php the_content(''); ?>
<div class="postmeta">
<span class="left"><?php if (function_exists('the_tags')) the_tags('Tags: '); ?></span>
<span class="right"><?php comments_popup_link('No Comments', '1 Comment', '% Comments', 'comments'); ?></span>
</div>
</div>

<?php endwhile; ?>