JSFiddle - React, Tailwind, and code Playground

HTML

<p>&nbsp;1</p>
<p>&nbsp;2</p>
<p>&nbsp;3</p>
<p>&nbsp;4</p>
<p>&nbsp;5</p>
<p>&nbsp;6</p>
<p>&nbsp;7</p>

JavaScript

$('p').each(function(i) {
    var pos = i + 1;
    console.log(pos, pos % 3, this)
    if (pos % 3 == 0) {
        $('<div/>', {
            class: 'anuncio',
            text: 'Div inserida!'
        }).insertAfter(this);
    }
});