We can use justify to make a group of buttons stretch at the same size to span the entire width of its parent.
<!DOCTYPE HTML>
<html>
<head>
<link href="http://java2s.com/style/bootstrap.min.css" rel="stylesheet">
<!--from w w w.j a va2s.c o m-->
<script src="http://java2s.com/style/jquery-1.8.0.min.js"></script>
<script src="http://java2s.com/style/bootstrap.min.js"></script>
</head>
<body style='margin:20px;'>
<div class="btn-group btn-group-justified">
<a href="#" class="btn btn-default">Left</a>
<a href="#" class="btn btn-default">Middle</a>
<a href="#" class="btn btn-default">Right</a>
</div>
</body>
</html>