<div class="container">
	<section class="item item1">This is the first box in the markup</section>
	<section class="item item2">This is the second box in the markup</section>
	<section class="item item3">This is the third box in the markup</section>
</div>

.container{
	background: mistyrose;
	display: flex;
	padding: 10px;
}
.item{
	flex:1;
	padding: 30px;
	color: black;
}
.item1{
	order: 3;
	background: tomato;
}
.item2{
	order: 1;
	background: lightgreen
}
.item3{
	order: 2;
	background: rosybrown;
}