Wordpress Loop: how to wrap each 3 posts into a div?

Thanks for your support guys! :) I tried both solutions but didn't work,I ended up with this 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>

I haven't tested it with wordpress so i'm not 100% sure it'll work. The idea is to use the modulus operator (see an example that matches your needs 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; ?>

<?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; ?>