.box:after {
    display:block;
    content:"";
    width: 30%;
    height:0px;
}

全部代码
.box {
    display: flex;
    flex-wrap: wrap;
    justify-content: space-between;
}
.item {
    width: 30%;
    height: 50px;
    background-color: #f1f8ff;
    margin-bottom: 10px;
}
.placeholder {
    width: 30%;
    height: 0px;
}

<div class="box">
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="item"></div>
    <div class="placeholder"></div>
</div>