[twitter-bootstrap-3] Bootstrap 3: Offset isn't working?

There is no col-??-offset-0. All "rows" assume there is no offset unless it has been specified. I think you are wanting 3 rows on a small screen and 1 row on a medium screen.

To get the result I believe you are looking for try this:

<div class="container">
    <div class="row">
      <div class="col-sm-4 col-md-12">
        <p>On small screen there are 3 rows, and on a medium screen 1 row</p>
      </div>
      <div class="col-sm-4 col-md-12">
        <p>On small screen there are 3 rows, and on a medium screen 1 row</p>
      </div>
      <div class="col-sm-4 col-md-12">
        <p>On small screen there are 3 rows, and on a medium screen 1 row</p>
      </div>
    </div>
  </div>

Keep in mind you will only see a difference on a small tablet with what you described. Medium, large, and extra small screens the columns are spanning 12.

Hope this helps.