<div class="btn-group btn-group-justified">
    <div class="btn-group">
        <button type="button" class="btn btn-default">Left</button>
    </div>
    <div class="btn-group">
        <button type="button" class="btn btn-default">Middle</button>
    </div>
    <div class="btn-group">
        <button type="button" class="btn btn-default">Right</button>
    </div>
</div>