How display multi thumbnails on post?

Hi

I use custom field for display image thumbnail ( thumbnail 1) of post. but now i want to add more thumbnails ( thumbnail 2, thumbnail 3) of post to beside thumbnail 1.
My code in index.php :

<div class="thumbnail">
<?php get_thumbnail($post->ID, 'thumbnail', 'alt="' . $post->post_title . '"'); ?>
</div>

and code in functions.php :

[Code moderated as per the Forum Rules. Please use the pastebin]

Please help me to solve this problem.
Thanks & Regards,