flex-direction 主軸の向きを決める

<div class="col">
  <span>1</span>
  <span>2</span>
  <span>3</span>
</div>

.col {
  display: flex;
  flex-direction: column;
  gap: 0.4rem;
}
.col > span {
  background: #efeef8;
  padding: 0.4rem 0.65rem;
}