JSFiddle - React, Tailwind, and code Playground

HTML

<div class="select-all-these except-these">1</div>
<div class="select-all-these except-these">2</div>
<div class="select-all-these except-these">3</div>
<div class="select-all-these">4</div>
<div class="select-all-these except-these">5</div>
<div class="select-all-these">6</div>
<div class="select-all-these">7</div>
<div class="select-all-these except-these">8</div>
<div class="select-all-these">9</div>
<div class="select-all-these">10</div>
<div class="select-all-these">11</div>

JavaScript

var count = 0;
$('.select-all-these').each(function() {
    if(!$(this).hasClass('except-these')) {
        count++;
    }
    if(count === 3) {
        $(this).text('every 3rd element');
        count = 0
    }
});