To use justified button groups with <button> elements, you must wrap each button in a button group. Most browsers don't properly apply our CSS for justification to <button> elements, but since we support button dropdowns, we can work around that.
<div class="btn-group btn-group-justified">
<div class="btn-group">
<button class="btn btn-default">Left</button>
</div>
<div class="btn-group">
<button class="btn btn-default">Middle</button>
</div>
<div class="btn-group">
<button class="btn btn-default">Right</button>
</div>
</div>