I can't imagine there is any advantage to a third shock on a cmx kit. Might actually be a detriment. The uncoupled skid will give you as much ski lift as you could ever want. On low snow days, I keep the preload backed off on the rear most shock and wheelie with the blip of a throttle. On deep days, I crank it up a bit to give better deep snow and climbing ability. You can adjust the preload to give you whatever personality you want the kit to have on any given day.
If a third shock was available, I sure wouldn't pay for it. Even if it was a no cost option, I'd only consider it if it had a lockout. Or more likely, I'd first ride one and if I saw no clear and extreme advantage, I'd stick with the simplicity and lighter weight of a fixed rod.