bootstrap justify-content-center for md
<div class="row justify-content-md-center"></div>
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>
display flex usnig bootstrap
<div class="d-flex p-2 bd-highlight">I'm a flexbox container!</div>