Bootstrap rows always contain their floats and create new lines. You don’t need to worry about filling blank columns, just make sure they don’t add up to more than 12.
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/3.2.0/css/bootstrap.min.css">
<div class="container">
<div class="row">
<div class="col-xs-3 col-xs-offset-9">
I'm a right column of 3
</div>
</div>
<div class="row">
<div class="col-xs-3">
I'm a left column of 3
</div>
</div>
<div class="panel panel-default">
<div class="panel-body">
And I'm some content below both columns
</div>
</div>
</div>