//Save the context in a var
	var $main = $("#mainContent");
	//If the container exists
	if($(".ngg-albumoverview").length){
		//Loop through each image, including the index of the object(i)
		$(".ngg-album", $main).each(function(i){
			//Since 0 based add one, for every 3rd object remainder will equal 0
			var remainder = (i + 1) % 3;
			//Add a class of last when the remainder is 0
			if(remainder === 0){
				$(this).addClass("last");
			}
		});
	}

//Save the context in a var
	var $main = $("#mainContent");
	//If the container exists
	if($(".ngg-albumoverview").length){
		//Loop through each image, including the index of the object
		$(".ngg-album", $main).each(function(i){
			//Since 0 based add one, for every 3rd object remainder will equal 0
			var remainder = (i + 1) % 3;
			//Add a class of last when the remainder is 0
			if(remainder === 0){
				$(this).addClass("last");
			}
		});
	}

Quick little clientside script to pick out every 3rd image and add a class of last to it.

I needed this because the images were layed out in 3 columns, adding a margin to the right hand side would leave a large gap on the right hand side which i didn't want. This now allows me to remove the margin for the last image.

Add class of 'last' to every 3rd image

jquery

jQuery