This is my code. I am using express.js. this is ejs file. the snapshot of code below is this 
<%- include ("../partials/header") %>
<div class="container">
<header class="jumbotron">
<div class="container">
<h1>Welcome To YelpCamp!</h1>
<p>View our hand-picked campgrounds from all over the world</p>
<p>
<a class="btn btn-lg btn-primary" href="/campgrounds/new">Add new Campground </a>
</p>
</div>
</header>
<div class="row" style="display:flex; flex-wrap:wrap;">
<% campgrounds.forEach(function(campground){ %>
<div class="col-md-3 col-sm-6">
<div class="thumbnail">
<img src="<%= campground.image %>">
<div class="caption">
<h4> <%= campground.name %> </h4>
</div>
<p>
<a href="/campgrounds/<%= campground._id %>" class="btn btn-primary">More Info</a>
</p>
</div>
</div>
<% }); %>
</div>
</div>
<%- include ("../partials/footer") %>
I want to adjust thumbnails to occupy empty spaces in between as it is looking hideous. is there any way to do that using bootstrap 3 ( I have also marked in image please refer the image) Any help is appreciated..