SOURCE

<div class="bd">
    <div class="left">left</div>
    <div class="main">main</div>
    <div class="right">right</div>
</div>

.bd{
    width:100%;
    height:400px;
    display: flex;
}
.left,.right{
    width:100px;
    background-color: yellow;
}
.main{
    flex:1;
    background-color: greenyellow;
}