unique id each post

unique-class-id.php

<?php $post_counter = 0; // start a counter before the loop ?>
<?php if ( have_posts() ) : ?>
    <?php while ( have_posts() ) : the_post(); ?>
    <?php $post_counter++; // increment the counter for each post ?>

        <article <?php post_class(); ?> id="layout-<?php echo $post_counter; ?>">

        <?php if ( $post_counter <= 3 ) : ?>
        // display something only for the third post in the loop
        <?php endif; ?>

        // the loop content goes here
        </article>

    <?php endwhile; ?>
<?php else : ?>
    // not found page
<?php endif; ?>