Wordpress loop : get current post count inside The Loop
You can use the current_post member of the WP_Query object instance to get the current post iteration;
while ( have_posts() ) : the_post(); // your normal post code if ( ( $wp_query->current_post + 1 ) % 3 === 0 ) { // your ad code here }endwhile;Note, if you're using this inside a function, you'll need to globalise $wp_query.