// in block.tpl.php

<?php 
  $block_order = 'block-left-middle ';
  if( $block_id == 1 ) {
  	$block_order = 'block-left-top ';
  } elseif( $block_id == count(block_list('left')) ) {
  	$block_order = 'block-left-bottom ';
  }
?>
<div id="block-<?php print $block->module .'-'. $block->delta; ?>" class="block <?php print $block_order . $block_classes; ?>">
 <div class="block-inner"> ...