The following columns start at 50% wide on mobile and bump up to 33.3% wide on desktop.
<!DOCTYPE HTML>
<html>
<head>
<link href="http://java2s.com/style/bootstrap.min.css" rel="stylesheet">
<style>div{background:#eee;border:1px solid #DDD;}</style>
</head><!--from w ww . j a v a2 s . c o m-->
<body style='margin:20px;'>
<div class="row">
<div class="col-6 col-lg-4">4</div>
<div class="col-6 col-lg-4">4</div>
<div class="col-6 col-lg-4">4</div>
</div>
</body>
</html>