Check out example codes for "col-xs-12 col-sm-4 col-md-4 col-lg-4". It will help you in understanding the concepts better.

Code Example 1

<div class="row">
  <div class="col-sm-4">.col-sm-4</div>
  <div class="col-sm-4">.col-sm-4</div>
  <div class="col-sm-4">.col-sm-4</div>
</div>

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