<div class="container">
  <div class="item">1</div>
  <div class="item">2</div>
  <div class="item">3</div>
  <div class="item">4</div>
  <div class="item">5</div>
  <div class="item">6</div>
  <div class="item">7</div>
  <div class="item">8</div>
  <div class="item">9</div>
  <div class="item">10</div>
  <div class="item">11</div>
  <div class="item">12</div>
  <div class="item">13</div>
  <div class="item">14</div>
  <div class="item">15</div>
  <div class="item">16</div>
  <div class="item">17</div>
  <div class="item">18</div>
</div>

 .container {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  grid-template-rows: repeat(6, 1fr);
  grid-auto-flow: column;
  gap: 10px;
}

.item {
  text-align:center;
  font-size: 1.1em;
  padding: 1.5em;
  color: black;
  background:#e2e2e2;
}