JSFiddle - React, Tailwind, and code Playground

HTML

<div class="pricetable">
    <div class="righttable">
        <div class="line">
            <span class="item">1</span>
            <span class="item">2</span>
            <span class="item">3</span>
        </div>
        <div class="line">
            <span class="item">1</span>
            <span class="item">2</span>
            <span class="item">3</span>
        </div>
        <div class="line">
            <span class="item">1</span>
            <span class="item">2</span>
            <span class="item">3</span>
        </div>
    </div>
</div>

CSS

.active { border: 2px red solid; }

JavaScript

var index = 3; // the third "item" element 
$(".pricetable .righttable .line:not(.blank) .item:nth-child(" + index + ")")
  .addClass("active");