CodePen Embed - QWWKxJZ

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

.box { 
  display: flex; 
  background: #eee;
}
.box-item{
	background: red;
	height: 50px;
	width: 50px;
	margin:10px; 
}
._right { 
  margin-left: auto; /* ←これ */ 
}

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