二个固定定位，用margin 把左边和右边都让出来。
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>三列浮动中间宽度自适应</title>
<style>
*{
margin: 0;
padding: 0;
}
#left{
border :1px solid red;
200px;
/* height: 200px; */
height: 100%;
position: absolute;
top :0px;
left :0px;
}
#right{
border :1px solid green;
200px;
/* height: 200px; */
height: 100%;
position: absolute;
top :0px;
right :0px;
}
#center{
background-color: #fcc;
border:1px solid black;
/* height: 200px; */
height: 100%;
margin-left:205px;
margin-right:205px;
}
.box{
height: 500px;
border:1px solid green;
position: relative;
}
</style>
</head>
<body>
<div class="box">
<div id="left"></div>
<div id="center"></div>
<div id="right"></div>
</div>
</body>
</html>