Did you set this up as 3 Corridor Models or 3 Material Layers in the same Corridor Model. I assume the latter because if they were in separate models one model could not affect the others at all. If 3 Material Layers it sounds like you may have some elements tagged twice for Material Layer - ie some Wall Top Elements are also in the Wall Base Material Layer - if you do not double Tag them then you should get just what you want
Go to Properties of the Corridor Surface Model and change the Material Layer that it is showing to the one that you want to explode. This should show you exactly what you are going to get when you explode, and will be the surface that you select to explode and it should only give you elements that are a part of that Material Layer.
In the Corridor Instructions, you can also if needed create a Material Layer for the surface that you want to create, you can then if needed double tag the elements that make up that layer / surface with this new special layer and then view that new special layer in the View before exploding it.
If you dont get it worked out - share the file and I will take a look latest Monday am if not before