<style>
  .box {
    display: flex;
  }

  .item {
    height: 100px;
    width: 100px;
    border: 1px solid green;
    color: green;
    font-size: 50px;
    line-height: 100px;
    text-align: center;
  }
</style>


<div class="box">
    <div class="item">1</div>
    <div class="item">2</div>
    <div class="item item-right">3</div>
</div>

.item-right {
    margin-left: auto;
  }