SOURCE

<div id="phone">
  <div id="box">
    <div id="iteml">
      <h1>iteml</h1>
    </div>
    <div id="item2">
      <h1>item2</h1>
    </div>
    <div id="item3">
      <h1>item3</h1>
    </div>
  </div>
</div>

#phone {
  border: 2px solid red;
  width: 375px;
  height: 667px;
}
#box {
  display: flex;
  flex-direction: row;
  flex-wrap: space-between;
}