<link href="//maxcdn.bootstrapcdn.com/bootstrap/3.3.0/css/bootstrap.min.css" rel="stylesheet" id="bootstrap-css">
<script src="//maxcdn.bootstrapcdn.com/bootstrap/3.3.0/js/bootstrap.min.js"></script>
<script src="//cdnjs.cloudflare.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script>
<div class="panel-group" id="accordion1">
<div class="panel panel-default">
<div class="panel-heading">
<h4 class="panel-title">
<a data-toggle="collapse" data-parent="#accordion1" href="#collapseThree">Collapsible Group Item #3
</a>
</h4>
</div>
<div id="collapseThree" class="panel-collapse collapse">
<div class="panel-body">
<div class="panel-group" id="accordion2">
<div class="panel panel-default">
<div class="panel-heading">
<h4 class="panel-title">
<a data-toggle="collapse" data-parent="#accordion2" href="#collapseThreeOne">Collapsible Group Item #3.1
</a>
</h4>
</div>
<div id="collapseThreeOne" class="panel-collapse collapse">
<div class="panel-body">Panel 3.1</div>
</div>
</div>
<div class="panel panel-default">
<div class="panel-heading">
<h4 class="panel-title">
<a data-toggle="collapse" data-parent="#accordion2" href="#collapseThreeTwo">Collapsible Group Item #3.2
</a>
</h4>
</div>
