<div class="container">
  <div class="row">
    <div class="col-sm-3">
      3つの列の1列目<br>
      .col-sm-3
    </div>
    <div class="col-sm-6">
      3つの列の2列目<br>
      .col-sm-6
    </div>
    <div class="col-sm-3">
      3つの列の3列目<br>
      .col-sm-3
    </div>
  </div>
</div>

.row > [class*=col] {
    padding-top: .75rem;
    padding-bottom: .75rem;
    background-color: rgba(86,61,124,.15);
    border: 1px solid rgba(86,61,124,.2);
}

.row {
  margin-top: 1rem;
}