How to create this design-loop?

This is untested but should work, I use something like this for a single category page?

in the style.css, you may need to fix the width in px or change to 49%, if not working in all browsers.

.two-column
{
	float:left;
	position: relative;
	width: 50%;
}

Not sure what theme you are using, code would sit inside the loop add a counter.

<!-- Start the Loop. -->
<?php if ( have_posts() ) : ?>
<?php while ( have_posts() ) : the_post(); ?>

<?php $counter+=1 ; ?>
<?php if($counter > 3): ?>
   <div class="two-column">
<?php endif; ?>

Then the loop output code, as you are using narrow columns then maybe output the excerpt if counter is greater than 3, let you work that out!

Just before the <?php endwhile; ?>

<?php if($counter > 3): ?>
   </div>
<?php endif; ?>
<?php endwhile; ?>

HTH

David

Add the style in the post above to your style.css

Twenty Ten normal:
Save loop.php as backup, create new loop.php then paste in this code:

http://adeptris.pastebin.ca/1929766

Twenty Ten Child Theme:
Create a new file loop-index.php, paste the same code inside the file.

You then have three posts with full width details, the two columns of post excerpts.

Changes:
View Pastebin Code
Lines: 58 – 61: Start Condition if counter > 3 adds the Div element
Lines: 144 – 148: Output excerpt if counter > 3
Lines: 179 – 181: Close the div if counter > 3

HTH

David