从ios中的epub获取封面图片(fetch Cover Image from epub in ios)

 您可以将col-sm-6更改为col-xs-6 （它将适用于x小屏幕和小屏幕） 
 
 然后删除col-lg-4因为它不需要（类col-md-4适用于从中等到所有更大的尺寸，除非另有说明） 
 
 完成后，你可以简单地在最后一列添加一个偏移量，如下所示： class="col-xs-6 col-xs-offset-3 col-md-4 col-md-offset-0" 
 
 基本上，如果xs偏移3列（然后将其居中），它将删除中等或更高屏幕的偏移量。 
 
 见下面的演示 
 
 
 
 
 
  
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<!-- Latest compiled and minified CSS -->
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" crossorigin="anonymous">

<!-- Optional theme -->
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap-theme.min.css" integrity="sha384-rHyoN1iRsVXV4nD0JutlnGaslCJuC7uwjduW9SVrLvRYooPp2bWYgmgJQIXwl/Sp" crossorigin="anonymous">

<!-- Latest compiled and minified JavaScript -->
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js" integrity="sha384-Tc5IQib027qvyjSMfHjOMaLkfuWVxZxUPnCJA7l2mCWNIpG9mGCD8wGNIcPD7Txa" crossorigin="anonymous"></script>

<div class="row">
  <div class="col-xs-6 col-md-4">
    <figure class="box-1">
      <img class="services-imgs" src="http://lorempixel.com/300/300" alt="rehab" />
      <figcaption class="figcaption"><a href="serviços.html">Reabilitação Vestibular</a>
      </figcaption>
    </figure>
  </div>
  <div class="col-xs-6 col-md-4">
    <figure class="box-1">
      <img class="services-imgs" src="http://lorempixel.com/300/300" alt="drenagem" />
      <figcaption class="figcaption"><a href="serviços.html">Fisioterapia Estética</a>
      </figcaption>
    </figure>
  </div>
  <div class="col-xs-6 col-xs-offset-3 col-md-4 col-md-offset-0">
    <figure class="box-1">
      <img class="services-imgs" src="http://lorempixel.com/300/300" alt="traumato-ortopédica" />
      <figcaption class="figcaption"><a href="serviços.html">Fisioterapia Traumato-Ortopédica</a>
      </figcaption>
    </figure>
  </div>
</div>
 
 
 
 

You can change col-sm-6 to col-xs-6 (it will apply to x-small and small screens)
 
Then remove the col-lg-4 as it is not needed (the class col-md-4 applies from medium to all bigger sizes, unless specified differently)
 
Having done that, you can simply add an offset to the last column like this: class="col-xs-6 col-xs-offset-3 col-md-4 col-md-offset-0"
 
Basically, if xs offset it by 3 columns (which then centers it) and it removes the offset for medium or higher screens.
 
See demo below
 
 
 
 
 
  
<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>
<!-- Latest compiled and minified CSS -->
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap.min.css" integrity="sha384-BVYiiSIFeK1dGmJRAkycuHAHRg32OmUcww7on3RYdg4Va+PmSTsz/K68vbdEjh4u" crossorigin="anonymous">

<!-- Optional theme -->
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/css/bootstrap-theme.min.css" integrity="sha384-rHyoN1iRsVXV4nD0JutlnGaslCJuC7uwjduW9SVrLvRYooPp2bWYgmgJQIXwl/Sp" crossorigin="anonymous">

<!-- Latest compiled and minified JavaScript -->
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.7/js/bootstrap.min.js" integrity="sha384-Tc5IQib027qvyjSMfHjOMaLkfuWVxZxUPnCJA7l2mCWNIpG9mGCD8wGNIcPD7Txa" crossorigin="anonymous"></script>

<div class="row">
  <div class="col-xs-6 col-md-4">
    <figure class="box-1">
      <img class="services-imgs" src="http://lorempixel.com/300/300" alt="rehab" />
      <figcaption class="figcaption"><a href="serviços.html">Reabilitação Vestibular</a>
      </figcaption>
    </figure>
  </div>
  <div class="col-xs-6 col-md-4">
    <figure class="box-1">
      <img class="services-imgs" src="http://lorempixel.com/300/300" alt="drenagem" />
      <figcaption class="figcaption"><a href="serviços.html">Fisioterapia Estética</a>
      </figcaption>
    </figure>
  </div>
  <div class="col-xs-6 col-xs-offset-3 col-md-4 col-md-offset-0">
    <figure class="box-1">
      <img class="services-imgs" src="http://lorempixel.com/300/300" alt="traumato-ortopédica" />
      <figcaption class="figcaption"><a href="serviços.html">Fisioterapia Traumato-Ortopédica</a>
      </figcaption>
    </figure>
  </div>
</div>