obviously, if you edit index.php, this might effect other pages as well such as archives, etc.
try and edit this section:
<?php if (have_posts()) : ?>
<?php while (have_posts()) : the_post(); ?>
<div <?php post_class() ?> id="post-<?php the_ID(); ?>">
using the idea of my link, change it to:
<?php if (have_posts()) : ?>
<?php $count = 0; ?>
<?php while (have_posts()) : the_post(); ?>
<?php $first = ($count%3 == 0) ? 'first' : ''; $count++; ?>
<div <?php post_class($first) ?> id="post-<?php the_ID(); ?>">
and add to style.css (example only, needs to be fine-tuned):
.post.first { float:left; clear:left; width: 30%; margin-left: 0; }
.post float:left; width: 30%; margin-left: 3%; }
http://codex.wordpress.org/Function_Reference/post_class
http://php.net/manual/en/language.operators.comparison.php