add a class to every second and third post

add a specific class to every second and third post in a group of 4 posts?

assuming that the post’s div has post_class(), this is doable with using the $wp_query->current_post of a default loop, in a filter function added to functions.php of your theme:

function special_recurrent_post_class($classes) {
  if( is_home() ) {
  global $wp_query;
  $extra_classes = array('','specific','specific','');
  $classes[] = $extra_classes[$wp_query->current_post%count($extra_classes)];
  }
  return $classes;
}
add_filter('post_class', 'special_recurrent_post_class');

(example restricts this to posts on the home page or posts page; for other, you need to change the coditional tag is_home() to something else; http://codex.wordpress.org/Conditional_Tags

watch out:
the code is for a group of 4;
your example shows a group of 3 – be sure about what you want.

if this is not what you want, please post the existing code of the template – see forum guidelines http://codex.wordpress.org/Forum_Welcome#Posting_Code