JSFiddle - React, Tailwind, and code Playground

HTML

<select name="dropdown[]">
  <option value="1">Sony</option>
  <option value="2">Nintendo</option>
  <option value="3">Microsoft</option>
</select>

<select name="dropdown[]">
  <option value="1">Sony</option>
  <option value="2">Nintendo</option>
  <option value="3">Microsoft</option>
</select>

<select name="dropdown[]">
  <option value="1">Sony</option>
  <option value="2">Nintendo</option>
  <option value="3">Microsoft</option>
</select>

JavaScript

var selects = document.querySelectorAll('select[name="dropdown[]"]');

selects[0].addEventListener('change', function () {
    for (var i = 0; i < selects.length; i++) {
        selects[i].value = selects[0].value;
    }
});