SOURCE

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

* {
    padding: 0;
    margin: 0;
}
.page {
    height: 200px;
    display: flex;
}
.left, .right {
    height: 200px;
    width: 200px;
    background-color: pink;
}
.content {
    height: 200px;
    background-color: aliceblue;
}

.content {
    flex: 1;
    order: 1;
    background-color: aliceblue;
}
.right {
    order: 2;
}