twbs4 vertical col spacing

HTML

<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/css/bootstrap.min.css">
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0-alpha.6/js/bootstrap.min.js"></script>
<body>

<h1 class="display-3">
hey
</h1>

  <div class="container">

    <div class="row">

      <div class="col-md-4 col-lg-4">
        col 1
      </div>

      <div class="col-md-4 col-lg-4">
        col 2
      </div>

      <div class="col-md-4 col-lg-4">
        col 3
      </div>

      <div class="col-md-4 col-lg-4">
        col 4
      </div>

      <div class="col-md-4 col-lg-4">
        col 5
      </div>

      <div class="col-md-4 col-lg-4">
        col 6
      </div>

    </div>

  </div>

</body>

CSS

[class*="col-"]:not(:last-child){
  margin-bottom: 15px;
}