某布局

<div class="wrap">
    <div class="main">
        <div class="mainin">main</div>
    </div>
    <div class="left">left</div>
    <div class="right">right</div>
</div>
.main,
    .left,
    .right {
        float: left;
        height: 200px;
    }
    
    .main {
         100%;
        background: #ace;
    }
    
    .left {
         20%;
        background: #eee;
        margin-left: -100%;
    }
    
    .right {
         30%;
        background: #ddd;
        margin-left: -30%;
    }
    
    .mainin {
        margin: 0 30% 0 20%;
    }