I am trying this:

<?php query_posts('cat=6'); ?>
<?php if (have_posts()) : ?>

    <?php while (have_posts()) : the_post(); ?>
        <div>
            <?php $counter=3; ?>
            <?php the_post_thumbnail(); ?>
            <?php $counter++; ?>
        </div>

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

But it is no longer working! :/ Thanks!

Interesting support men! :) I attempted both solutions but did not work, I wound up with this particular and works perfectly!

<?php query_posts('cat=6'); ?>

<?php $variable=0;?>

<div>
<?php while ( have_posts() ) : the_post(); ?>
<?php if(($variable+1)<4){ ?>
<a href="<?php echo get_post_meta($post->ID, 'colaborador-link', true); ?>" target="blank">
<?php the_post_thumbnail(); ?>
</a>
<?php $variable+=1; ?>
<?php }else{ ?>
<?php $variable=1; ?>
</div>

<div>
<a href="<?php echo get_post_meta($post->ID, 'colaborador-link', true); ?>" target="blank">
<?php the_post_thumbnail(); ?>
</a>
<?php }?>
<?php endwhile; ?>
</div>
<?php query_posts('cat=6'); ?>
<?php if (have_posts()) : ?>
<?php $counter=0; ?>
<?php while (have_posts()) : the_post(); ?>
<?php if($counter%3==0) : ?>        
<div>
        <?php $counter=3; ?>
        <?php the_post_thumbnail(); ?>
        <?php $counter++; ?>
</div>
<?php else: 
//Some code for other posts..
endif;
?>
<?php $counter++; ?>
<?php endwhile; ?>
<?php endif; ?>

I've not examined it with wordpress so i am not 100% sure it'll work. The concept is by using the modulus operator (see a good example that feels like a fit here http://codepad.org/78d2aAKp)

<?php query_posts('cat=6'); ?>
<?php if (have_posts()) : ?>
<!-- Your div starts here -->
<div>
<?php 
    while (have_posts()) : 
        the_post();
        $counter = 0;
        if($counter%3 == 0 && $counter > 0):
?>
<!--Close and then open the div-->
</div><div>
<?php 
        endif;
?>
<?php the_post_thumbnail(); ?>
<?php $counter++; ?>
<?php endwhile; ?>
</div><!--/Your div ends here -->
<?php endif; ?>