SOURCE

<div class="header">
    header
</div>

<div class="container">
    <div class="main">我是主要内容
        fldsjfl
        <p>fdfd

            fdfdfdfd

            dff
        </p>
    </div>
    <div class="left">我是左边</div>
    <div class="right">我是右边</div>
</div>

<div class="footer">
    footer
</div>

.header{
      text-align: center;
    line-height: 80px;
    height: 80px;
    background-color: blue;
}
.main,.left,.right{
    text-align: center;
    float: left;
    position: relative;
}

.main{
    width: 100%;
    background-color: gray;
}

.left{
    width: 100px;
    margin-left: -100%;
    left: -100px;
    background-color: darkgoldenrod;
    height: 100%;
}

.right{
    width: 100px;
    margin-left: -100px;
    right: -100px;
    background-color: orange;
}

.footer{
    text-align: center;
    line-height: 80px;
    height: 80px;
    background-color: red;
    clear: both;
}

.container{
    padding: 0 100px;
    /* min-width: 400px; */
}