wp_nav_menu, add class to every nth item?

is there a way to add a class to every nth item in a menu using wp_nav_menu?

Trying to create columns but need to add a ‘last’ class to the 3rd item.

cheers! Dc

You can use jQuery to do that. Try this in your header :

$(document).ready(function() {
    $("#mymenu li:nth-child(3n+3)").addClass("last");
});

Note : for this to work you need to have enqueued jQuery.