<style type="text/css">
#outer{
display: flex;
width: 100%;
flex-flow: row nowrap;
}
.left,.right{
width:200px;
height:50px;
background: red;
}
.middle{
flex-grow: 1; //決定如何分配剩余空間
background: yellow;
}
</style>
<body>
<div id="outer">
<div class="left">left</div>
<div class="middle">middle</div>
<div class="right">right</div>
</div>