JSFiddle - React, Tailwind, and code Playground

HTML

<ul>
    <li>1</li>
    <li>2</li>
    <li>3</li><!--target list item-->
    <li>4</li>
    <li>5</li>
    <li>6</li><!--target list item-->
    <li>7</li>
</ul>

CSS

.third {
    background-color: red;
}

JavaScript

$('li:nth-child(3n)').addClass('third');