创建行大小为3，列大小为N的div

<div class="container">
  <div class="row">
    <div class="col">1</div>
    <div class="col">2</div>
    <div class="col">3</div>
    <!-- 继续添加列直到N -->
  </div>
  <div class="row">
    <div class="col">1</div>
    <div class="col">2</div>
    <div class="col">3</div>
    <!-- 继续添加列直到N -->
  </div>
  <div class="row">
    <div class="col">1</div>
    <div class="col">2</div>
    <div class="col">3</div>
    <!-- 继续添加列直到N -->
  </div>
</div>

.container {
  display: flex;
  flex-direction: column;
}

.row {
  display: flex;
}

.col {
  flex: 1;
  border: 1px solid black;
  padding: 10px;
}