how use col-sm-6 for 3 element and centerd code example

Example 1: bootstrap grid 2 rows

<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.5/css/bootstrap.min.css" rel="stylesheet"/>


<div class="row">
    <div class="col-md-4">
        <div class="well">1
            <br/>
            <br/>
            <br/>
            <br/>
            <br/>
        </div>
    </div>
    <div class="col-md-8">
        <div class="row">
            <div class="col-md-6">
                <div class="well">2</div>
            </div>
            <div class="col-md-6">
                <div class="well">3</div>
            </div>
        </div>
        <div class="row">
            <div class="col-md-6">
                <div class="well">4</div>
            </div>
            <div class="col-md-6">
                <div class="well">5</div>
            </div>
        </div>
    </div>
</div>
<div class="row">
    <div class="col-md-4">
        <div class="well">6</div>
    </div>
    <div class="col-md-4">
        <div class="well">7</div>
    </div>
    <div class="col-md-4">
        <div class="well">8</div>
    </div>
</div>

Example 2: boot strap columns

<div class="container">
  <div class="row">
    <div class="col">
      1 of 3
    </div>
    <div class="col-6">
      2 of 3 (wider)
    </div>
    <div class="col">
      3 of 3
    </div>
  </div>
  <div class="row">
    <div class="col">
      1 of 3
    </div>
    <div class="col-5">
      2 of 3 (wider)
    </div>
    <div class="col">
      3 of 3
    </div>
  </div>
</div>