According to a video i watched that Michael Palmer done, he states that you can use a queen excluder on a double nuc with no problems and put honey supers above, as long as you make sure that there is no way the queens can get from one side to the other. The workers don't pay no attention to each other and work right away at filling up the super. Going based upon that i'd say that using a 3 queen castle system would work also. But they wouldn't be getting very strong as they would be limited to how many frames were in their side of the castle. He did state in the video that the only problem with that would be that when it came time to remove the super, the bees wouldn't have any particular alliance to a queen and could all end up on one side of the nuc.