JSFiddle - React, Tailwind, and code Playground

HTML

<body id="page-mod-quiz-attempt">
<div id="page-content">
<div class="col-md-3">

</div>
</div>
</body>

CSS

// Just for visual example
.col-md-3 {
  width: 100px;
  height: 100px;
  background-color: red;
}

.col-md-3.col-xs-12.col-sm-12 {
  width: 100px;
  height: 100px;
  background-color: green;
}
}

JavaScript

var Contenz = document.getElementById('page-content');
console.log(Contenz)

var Final = Contenz.getElementsByClassName('col-md-3')[0].className += ' col-xs-12 col-sm-12';
// console.log(Final); // returns col-md-3 col-xs-12 col-sm-12