如何每隔5秒随机淡入/淡出若干div？

<div id="div1" class="fade-in-out">Div 1</div>
<div id="div2" class="fade-in-out">Div 2</div>
<div id="div3" class="fade-in-out">Div 3</div>

.fade-in-out {
  opacity: 0;
  transition: opacity 1s ease-in-out;
}

.fade-in-out.fade-in {
  opacity: 1;
}

.fade-in-out.fade-out {
  opacity: 0;
}

function fadeRandomDiv() {
  var divs = document.getElementsByClassName('fade-in-out');
  var randomIndex = Math.floor(Math.random() * divs.length);
  var randomDiv = divs[randomIndex];

  randomDiv.classList.add('fade-in');
  setTimeout(function() {
    randomDiv.classList.remove('fade-in');
    randomDiv.classList.add('fade-out');
    setTimeout(function() {
      randomDiv.classList.remove('fade-out');
    }, 1000);
  }, 4000);
}

setInterval(fadeRandomDiv, 5000);