basic 3 col loop

<div id="column-wrap">
                <?php
            $count = 0;

            while(have_posts()) : the_post()
        ?>

      <div class="box<?php if( $count%3 == 0 ) { echo '-1'; }; $count++; ?>">

      <a href="<?php the_permalink(); ?>">
      <h4><?php the_title(); ?></h4></a>
      <!--and other output of the loop -->

      </div>

         <?php endwhile; ?>
</div><!--end column-wrap-->

----------------
STYLES - EXAMPLE

.box-1 { float:left; clear:left; width: 30%; margin-left: 0; }
.box { float:left; width: 30%; margin-left: 3%; }