malsyst,
<!DOCTYPE HTML>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
.clearfix.visible-sm-block {
border: 1px dashed Gray; padding: 5px;
}
.clearfix.visible-xs-block {
border: 1px dashed #FF0000; padding: 5px;
}
</style>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.12.0/jquery.min.js"></script>
<script>
$(function(){
$(".row.items").each(function(indx, el){
$(".item", el).each(function(i, elem){
!(++i % 2) && $("<div>",{"class": "clearfix visible-xs-block"}).insertAfter(elem);
!(i % 4) && $("<div>",{"class": "clearfix visible-sm-block"}).insertAfter(elem);
})
});
})
</script>
</head>
<body>
<div class="row items">
<div class="item col-xs-6 col-sm-4">Lorem ipsum dolor sit amet</div>
<div class="item col-xs-6 col-sm-4">Lorem ipsum dolor sit amet</div>
<div class="item col-xs-6 col-sm-4">Lorem ipsum dolor sit amet</div>
<div class="item col-xs-6 col-sm-4">Lorem ipsum dolor sit amet</div>
<div class="item col-xs-6 col-sm-4">Lorem ipsum dolor sit amet</div>
<div class="item col-xs-6 col-sm-4">Lorem ipsum dolor sit amet</div>
<div class="item col-xs-6 col-sm-4">Lorem ipsum dolor sit amet</div>
<div class="item col-xs-6 col-sm-4">Lorem ipsum dolor sit amet</div>
<div class="item col-xs-6 col-sm-4">Lorem ipsum dolor sit amet</div>
<div class="item col-xs-6 col-sm-4">Lorem ipsum dolor sit amet</div>
<div class="item col-xs-6 col-sm-4">Lorem ipsum dolor sit amet</div>
<div class="item col-xs-6 col-sm-4">Lorem ipsum dolor sit amet</div>
<div class="item col-xs-6 col-sm-4">Lorem ipsum dolor sit amet</div>
<div class="item col-xs-6 col-sm-4">Lorem ipsum dolor sit amet</div>
<div class="item col-xs-6 col-sm-4">Lorem ipsum dolor sit amet</div>
<div class="item col-xs-6 col-sm-4">Lorem ipsum dolor sit amet</div>
<!--и т.д -->
</div>
</body>
</html>