Bootstrap class: .nav with flex utils

<nav class="nav nav-pills flex-column flex-sm-row">
  <a class="flex-sm-fill text-sm-center nav-link active" href="#">Active</a>
  <a class="flex-sm-fill text-sm-center nav-link" href="#">Link</a>
  <a class="flex-sm-fill text-sm-center nav-link" href="#">Link</a>
</nav>

Preview

More in Bootstrap Navs