<div class="container">
  <div class="item item1">1</div>
  <div class="item item2">2</div>
  <div class="item item3">3</div>
</div>

<style>
  .container {
    display: flex;
    width: 300px;
    border: 1px solid #ccc;
  }
  .item {
    padding: 10px;
    color: white;
  }
  .item1 { flex-grow: 1; background: #007bff; }
  .item2 { flex-grow: 2; background: #28a745; }
  .item3 { flex-grow: 1; background: #dc3545; }
</style>