Twitter Boostrap 3 grid margin

Twitter Boostrap 3 grid margin

I have created following scaffold:
<div class="row">
<div class="col-md-4">
first column
</div>
<div class="col-md-4">
second column
</div>
<div class="col-md-4">
third column
</div>
</div>
Every column has background with 1px border. I would like to add
margin-right: 15px to every column bot if applied, third column goes lower
to the second `row`.
How can I set 15px space between columns ?