Twenty Twelve theme: How do I arrange posts into a grid

So I’ve got a site running http://alturl.com/sd8vm which is currently displaying as a grid. Works great in Chrome and Firefox, but not in IE due to not supporting nth child selectors (I am using 3n+3) to hide the margin on the third grid of each row.

So I’ve tried jQuery, Counter++ etc but can’t seem to get them to work as my knowledge is limited (and guides don’t cater for Twenty Twelve specifically).

Can someone basically post what I need to do to get this working (bearing in mind that my knowledge is limited to pretty much the basic stuff).

Thanks!