JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li>раз</li>
    <li>два</li>
    <li>три</li>
    <li>раз</li>
    <li>два</li>
    <li>три</li>
</ul>

CSS

ul{
    width: 300px;
    border: 1px solid #000;
}

li{
    float: left;
    width: 28%;
    margin: 0 8% 0 0;
}

JavaScript

$('li').each(function () {
      $("li:nth-child(3n+3)").css({'background' : 'red'});
    });