Bootstrap col-sm-6 3 items per line - css

Question

I have a container with a row and inside that six portfolios items.
This code when used on a small screen I get the following result:
1st line : 2 images
2nd line : 1 image on the right side
3rd line : 2 images
4th line : 1 image on the left side
Anyone got a clue? Seems like a bug to me.
<div class="container">
<div class="row">
<div class="col-md-4 col-sm-6 col-xs-12">
<div class="portfolio-thumb">
<img src="images/portfolio-img1.png" alt="image" class="img-responsive">
<div class="portfolio-overlay">
<h2>Project One</h2>
<a href="#">
<i class="fa fa-search"></i>
</a>
</div>
</div>
</div>
<div class="col-md-4 col-sm-6 col-xs-12">
<div class="portfolio-thumb">
<img src="images/portfolio-img2.png" alt="image" class="img-responsive">
<div class="portfolio-overlay">
<h2>Project One</h2>
<a href="#">
<i class="fa fa-search"></i>
</a>
</div>
</div>
</div>
<div class="col-md-4 col-sm-6 col-xs-12">
<div class="portfolio-thumb">
<img src="images/portfolio-img3.png" alt="image" class="img-responsive">
<div class="portfolio-overlay">
<h2>Project One</h2>
<a href="#">
<i class="fa fa-search"></i>
</a>
</div>
</div>
</div>
<div class="col-md-4 col-sm-6 col-xs-12">
<div class="portfolio-thumb">
<img src="images/portfolio-img4.png" alt="image" class="img-responsive">
<div class="portfolio-overlay">
<h2>Project One</h2>
<a href="#">
<i class="fa fa-search"></i>
</a>
</div>
</div>
</div>
<div class="col-md-4 col-sm-6 col-xs-12">
<div class="portfolio-thumb">
<img src="images/portfolio-img1.png" alt="image" class="img-responsive">
<div class="portfolio-overlay">
<h2>Project One</h2>
<a href="#">
<i class="fa fa-search"></i>
</a>
</div>
</div>
</div>
<div class="col-md-4 col-sm-6 col-xs-12">
<div class="portfolio-thumb">
<img src="images/portfolio-img1.png" alt="image" class="img-responsive">
<div class="portfolio-overlay">
<h2>Project One</h2>
<a href="#">
<i class="fa fa-search"></i>
</a>
</div>
</div>
</div>
</div>
</div>

Answer 1

This is due to the different height of each post image, to avoid this use same size images or set the height of your <div class="col-md-4 col-sm-6 col-xs-12">...</div> similar to each other.
You can use the background-image property instead of just using <img> on the <div>.
Solution:
.portfolio-thumb {
width: 300px;
height: 200px;
background-size: cover;
}
<link href="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.3.7/css/bootstrap.min.css" rel="stylesheet"/>
<div class="container">
<div class="row">
<div class="col-md-4 col-sm-6 col-xs-12">
<div class="portfolio-thumb" style="background-image: url('http://placehold.it/300x200');"></div>
<div class="portfolio-overlay">
<h2>Project One</h2>
<a href="#">
<i class="fa fa-search"></i>
</a>
</div>
</div>
<div class="col-md-4 col-sm-6 col-xs-12">
<div class="portfolio-thumb" style="background-image: url('http://placehold.it/300x200');"></div>
<div class="portfolio-overlay">
<h2>Project One</h2>
<a href="#">
<i class="fa fa-search"></i>
</a>
</div>
</div>
<div class="col-md-4 col-sm-6 col-xs-12">
<div class="portfolio-thumb" style="background-image: url('http://placehold.it/300x200');"></div>
<div class="portfolio-overlay">
<h2>Project One</h2>
<a href="#">
<i class="fa fa-search"></i>
</a>
</div>
</div>
<div class="col-md-4 col-sm-6 col-xs-12">
<div class="portfolio-thumb" style="background-image: url('http://placehold.it/300x200');"></div>
<div class="portfolio-overlay">
<h2>Project One</h2>
<a href="#">
<i class="fa fa-search"></i>
</a>
</div>
</div>
<div class="col-md-4 col-sm-6 col-xs-12">
<div class="portfolio-thumb" style="background-image: url('http://placehold.it/300x200');"></div>
<div class="portfolio-overlay">
<h2>Project One</h2>
<a href="#">
<i class="fa fa-search"></i>
</a>
</div>
</div>
<div class="col-md-4 col-sm-6 col-xs-12">
<div class="portfolio-thumb" style="background-image: url('http://placehold.it/300x200');"></div>
<div class="portfolio-overlay">
<h2>Project One</h2>
<a href="#">
<i class="fa fa-search"></i>
</a>
</div>
</div>
</div>
</div>
Hope this helps!