query_posts() function acts weird… need help!

Dear WP gurus!

Please help me to clarify:

when I’m trying to show posts from a single or multiple categories using this in the page code:

<?php query_posts(cat=’3′);> or <?php query_posts(cat=’3,4,5′);>

as a result I have a completely blank page???

But when I excluding the category like this:

<?php query_posts(cat=’-4′);>

as a result I have all posts except the posts from category with ID 4, as I think it works perfect…
but why then WP generates blank page when I tell him to show category with a specified ID??

Here is a complete loop code it’s quite simple:

<?php query_posts(‘cat=3’) ?>
<?php if (have_posts()) : ?>
<?php while (have_posts()) : the_post(); ?>

<!–LOOP CONTENT–>

<?php endwhile; ?>
<?php endif; ?>

Thnx in advance for help!