<!-- HTML inputs -->
<input class="input required" type="text" placeholder="fill me">
<input class="input" type="text" placeholder="fill me">
<input class="input required" type="text" placeholder="fill me">
<button class="button">Click Me</button>

<!-- Js -->
let inputs = document.querySelectorAll('.required'),
    button = document.querySelector('.button');
button.disabled = true;

for (i = 0; i < inputs.length; i++) {
  inputs[i].addEventListener('input',() => {
    let values = []
    inputs.forEach(v => values.push(v.value))
    button.disabled = values.includes('')
  })
}