Insert Custom “Entry/Advertisement” to the Blog Grid

add_action( 'wpex_hook_archive_loop_before_entry', function() {

	// Add extra content only for blog archives
	if ( 'blog' != wpex_get_index_loop_type() ) {
		return;
	}

	global $wp_query;

	// Insert after the 3rd item (note in WP the first item has an index of 0)a
	if ( 2 == $wp_query->current_post ) {

		// Add to global entry counter
		global $wpex_count;
		$wpex_count++;

		// Get classes for blog posts (need if you are inserting into a grid)
		$classes = wpex_blog_entry_classes();

		// Your custom content
		echo '<div class="' . implode( ' ', $classes ) . '">Custom item added after the first 3 posts</div>';

	}

} );