Something like this should work for you.
$args = array( 'numberposts' => 3 );
$lastposts = get_posts( $args );
$count = 1;
foreach($lastposts as $post) : setup_postdata($post); ?>
<div id="blog" class="<?php if ($count == 3) : ?>last<?php endif; ?>">
<div class="blog-header"><a href="https://wordpress.stackexchange.com/questions/34591/<?php the_permalink(); ?>">
<?php the_title(); ?>
</a>
<div class="date">
<?php the_date(); ?>
</div>
</div>
<?php the_excerpt(); ?>
</div>
<?php
$count++;
endforeach; ?>