How to mark every 3rd post

My approach. No extra function, no filter. 🙂

<?php $GLOBALS['wpdb']->current_post = 0; ?>
<div <?php post_class( 0 === ++$GLOBALS['wpdb']->current_post % 3 ? 'third' : '' ); ?>>

Alternative:

<div <?php post_class( 0 === ++$GLOBALS['wpdb']->wpse_post_counter % 3 ? 'third' : '' ); ?>>

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