Making IDs show up with 3-digits

A not so well known WP function will work here:

$id3 = zeroise($post->ID, 3);

Where exactly would I put this?

(I use the tag <?php the_ID(); ?> – from what I can understand of the code you gave, it seems to have a specific id in mind…

You can replace

<?php the_ID(); ?>

with

<?php echo zeroise($post->ID, 3); ?>

Ah, I see. Sorry.

Thank you!