Applying a class to the first and last post of each row!

I’m building a wodpress theme using the 960 grid system. I’ve got a startpage that displays three thumbnails on each row. These thumbnails are in a child grid with parent wrapper.

So, I need to wrap the first post of each row with a class called “alpha” and the last post (third in my case) of each row with a class called “omega”.

Any idea how I might solve this?

Here’s my current code, any help is appreciated!

<?php get_header(); ?>

<div class="grid_12 projects"><!-- PROJECTS BEGINS -->

<?php
if (have_posts()) :
while (have_posts()) :
?>

<div class="grid_4 project">

<?php
the_post();
?>

<a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>">
<?php the_post_thumbnail(); ?>
</a>

<h2>
<a href="<?php the_permalink(); ?>" title="<?php the_title_attribute(); ?>">
<?php the_title(); ?>
</a>
</h2>

</div><!-- PROJECT ENDS -->

<?php
endwhile;
endif;
?>

</div><!-- PROJECT-CONTAINER ENDS -->

<?php get_footer(); ?>