First you need to add default class="active"
to the first element.
Second you need to add data-toggle="tab"
to the <a />
element. See the docs
Your code becomes:
<ul id="menu_area" class="nav nav-pills nav-justified custom">
<li class="active"><a href="#knowing_us" data-toggle="tab" id="menu_text">one</a></li>
<li><a href="#knowing_us2" data-toggle="tab" id="menu_text">two</a></li>
<li><a href="#knowing_us3" data-toggle="tab" id="menu_text">three</a></li>
</ul>