<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<style>
.one{
width: 20%;
height: 40px;
background-color: red;
float: left;
}
.three{
width: 20%;
height: 40px;
background-color: black;
float: right;
opacity: 0.4;
}
.two{
width: 60%;
height: 40px;
background-color: yellow;
margin: 0 auto;
}
</style>
</head>
<body>
<!-- 先写左右两边 最后写中间 -->
<div class="one">1</div>
<div class="three">3</div>
<div class="two">2</div>
</body>
</html>