Edit in JSFiddle

$('a').click(function() {
    $('<div/>').css({
        'width': '100%',
        'clear': 'both',
        'height': '0',
        'border': '0'
    }).insertAfter('div:nth-child(3n)');
});
<div>1st</div>
<div class="longone">2nd</div>
<div>3rd</div>
<div>4th</div>
<div class="longone">5th</div>
<div class="shortone">6th</div>
<div>7th</div>

<a href="#">Apply selector</a>
div {border:1px red solid;float: left;height:80px; width:30%;}
div.longone {height: 160px;}
div.shortone {height: 40px;}

a {clear:both;display:block;}