JSFiddle - React, Tailwind, and code Playground

HTML

<form>
  <input type="checkbox" value="one" name="cb">
  <input type="checkbox" value="two" name="cb">
  <input type="checkbox" value="three" name="cb">

  <input type="text" name="text">
</form>

JavaScript

document.querySelector('form').onchange = function() {
	this.text.value = Array.from(this.cb)
  	.filter(item => item.checked)
    .map(item => item.value)
    .join(',');
}