Bootstrap CSS class .flex-*-column

<div class="d-flex flex-column">
  <div class="p-2">Flex item 1</div>
  <div class="p-2">Flex item 2</div>
  <div class="p-2">Flex item 3</div>
</div>

<!-- responsive variations -->
<div class="d-flex flex-sm-column">...</div>
<div class="d-flex flex-md-column">...</div>
<div class="d-flex flex-lg-column">...</div>
<div class="d-flex flex-xl-column">...</div>