Singles are usually for pre and post order in a drive thru. These can also be used in a forecourt in a service station. Pre and post order is where you can impact your customers decision making process for current and future orders. 

Doubles are typically used to showcase your menu boards at the order point. This is where your customer makes their final decision and may add any last minute items they see. Doubles are great for showcasing your menus in a smaller space.

Triples are also typically used at the order point, however triples can showcase a much greater selection of your menu and have a greater impact on last minute decisions, such as additional menu items.