Show a popup on every third page

every_third_page.php

<?php // Ignore this first line when copying to your child theme's functions.php file.

function every_third_page() { ?>
 	<script type="text/javascript">
		
		/** Set up a page counter in the browser's session storage. */
		(function(){
		  let pc = parseInt(sessionStorage.getItem("my-page-count"));
		  if (!pc) {
		  	sessionStorage.setItem("my-page-count", 1);
		  } else {
			sessionStorage.setItem("my-page-count", pc += 1);
		  }
		})();
		
		// Make sure the function (IIFE) above runs before the following jQuery.
		
		jQuery(document).ready(function ($) {
			// Display a popup on every "third" page. Change 2178 throughout to your popup's HTML ID.
			$("#pum-2178").on("pumBeforeOpen", function (e) { // Add a listener.
				let pc = parseInt(sessionStorage.getItem("my-page-count"));
				// You can change 3 to what you want. E.g., 2 opens on ever second page, 4 opens on every fourth page.
				if ( (pc % 3) == 0 ) return; // If we're on a "third" page, just return so the popup displays.
				PUM.preventOpen(2178); // Stop the popup from opening because we're not on a "third" page.
			}); // Listener
		}); // jQuery

 	</script>
<?php }
add_action( 'wp_footer', 'every_third_page', 500 );

/**
 * You can add the PHP code snippet to your child theme's functions.php file
 * or with third-party plugins such as My Custom Functions and Code Snippets.
 *
 * Learn more:
 * - https://docs.wppopupmaker.com/article/84-getting-started-with-custom-js
 * - https://docs.wppopupmaker.com/article/552-getting-started-with-custom-php
 * - https://docs.wppopupmaker.com/article/409-find-the-popup-id
 */