HTML

Bootstrap 4 Flex

<div class="d-flex p-3 bg-secondary text-white">
  <div class="p-2 bg-info">Flex item 1</div>
  <div class="p-2 bg-warning">Flex item 2</div>
  <div class="p-2 bg-primary">Flex item 3</div>
</div>

Bootstrap flex box

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

<div class="d-flex">
  <div class="mr-auto p-2">Flex item</div>
  <div class="p-2">Flex item</div>
  <div class="p-2">Flex item</div>
</div>

<div class="d-flex">
  <div class="p-2">Flex item</div>
  <div class="p-2">Flex item</div>
  <div class="ml-auto p-2">Flex item</div>
</div>

bootstrap display flex

<div class="d-flex">I'm a flexbox container!</div>