SOURCE

<div class="page">
    <div class="left">左广告位</div>
    <div class="content">主要内容</div>
    <div class="right">右广告位</div>
</div>

* {
    padding: 0;
    margin: 0;
}
.page {
    height: 200px;
    display: table;
    table-layout: fixed;
    width: 100vw;
}
.left, .right {
    height: 200px;
    width: 200px;
    background-color: pink;
}

.content {
    height: 200px;
    background-color: aqua;
    width: 100%;
}
.page div {
    display: table-cell;
}