<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title>Document</title>
<style>
.box{border: 2px dotted black;}
.box::after{clear: both; content: ''; display: block;}
.box>div{width: calc(100% / 3); height: 480px; box-sizing: border-box; float: left;}
.box1{background-color: blue;}
.box2{background-color: white;}
.box3{background-color: red;}
</style>
</head>
<body>
<div class="box">
<div class="box1"></div>
<div class="box2"></div>
<div class="box3"></div>
</div>
</body>
</html>