SOURCE

function getSize() {
  for (var i = 1; i < 4; i++) {
    document.getElementById('item' + i).innerText = document.getElementsByClassName('item' + i)[0].clientWidth;
  }
}

window.onresize = getSize;
getSize();

<div class="container">
  <div class="item item1">
    1
  </div>
  <div class="item item2">
    2
  </div>
  <div class="item item3">
    3
  </div>
</div>
<h3>
  item1的长度：
  <span id="item1">
  </span>
</h3>
<h3>
  item2的长度：
  <span id="item2">
  </span>
</h3>
<h3>
  item3的长度：
  <span id="item3">
  </span>
</h3>

html,
body {
  margin: 0;
  padding: 0;
}

.container {
  background-color: #ccc;
  display: flex;
}

.item {
  margin: 10px;
  line-height: 40px;
  text-align: center;
  font-size: 20px;
  background-color: #0099cc;
}

.item {
  width: 100px;
}

.item2 {
  /* flex-grow: 1; */
}

.item3 {
  /* flex-grow: 2; */
}


/*.item2, item3 {*/


/*flex-basis: 0;*/


/*}*/