Bootstrap columns stacking vertically on mobile device

Answer 1

The columns are stacking vertically because you're using Bootstrap 4, and the -xs- infix is no longer used. Just use col-4..

  <div class="container border-show center-div">
    <div class="row">
      <div class="col-4 border-show">1</div>
      <div class="col-4 border-show">2</div>
      <div class="col-4 border-show">3</div>
    </div>
  </div>

http://www.codeply.com/go/sCct2CzZte

Answer 2

I can't quite make out the question because the HTML you posted does exactly what you want it to do: It never stacks the columns vertically for that row.

Could you clarify what you're trying to ask? See How To Ask for a reference of how to ask good questions.