Inserting ads after 3rd paragraph

Hi,

On my site: https://www.imperiumromanum.edu.pl/ I want to add on each site after 3rd paragraph the same line of script code with ads. I googled solutions how to do that, but unluckily I cannot find any sure information.
I was trying to adapt this solution: https://www.maketecheasier.com/insert-ads-in-between-content-in-wordpress/ but without result.

I found that there should be made changes in file ‘functions.php’.
However in wp-content I can find three different files with the same name:

– 1st location: wp-content\themes\romanum\ex
– 2nd location: wp-content\themes\romanum
– 3rd location: wp-content\themes\romanum\ex\includes\xcrud

“romanum” is my used themes.

Second file is small with information:

<?php

require_once('ex/functions.php');

How should I input code:

<div style="margin: 0 auto; text-align: center; max-width: 100%;"><script type="text/javascript">

    (function(window, document, Adform){

        window._adform = window._adform || [];

        _adform.push(['5686686.on.init', function (settings) {

            var flashvars = settings.html.flashvars;

            flashvars.pmpId = 171041;

        }]);

    })(window, document, (Adform = window.Adform || {}));

</script>

<script data-adfscript="track.adform.net/adfscript/?bn=5686686">

(function(c,b,e,a,d){

    c.getElementById("adform-adf"+b)||(a=c.createElement(b),

    a.type="text/java"+b,a.async=a.defer=!0,a.id="adform-adf"+b,

    a.src="http"+e+"://s1.adform.net/banners/scripts/adfscript.js?"+Math.round(new Date/6E4),

    (d=c.getElementsByTagName(b)[0]).parentNode.insertBefore(a,d))

})(document,"script","https:"==location.protocol?"s":"");

</script></div>

Thank You for help.
Cosiek