I understand that it would be very difficult, but I think the real draw that separates Construct from other 3d engines is the simplicity of the interface mixed with all of the functions, which would definitely serve its purpose in a 3d environment. I realize it would almost be worth it to create a C++ game, but the visual interface would definitely help instead of having to write up all of the code to interact with DirectX9.0 which is one of the biggest challenges in making any sort of game, thus the simplicity in Construct.
I'm not really sure exactly the math behind the 3d functions, I mean I'm sure I've learnt a lot of the 3d functions and vectors and everything, but I don't really know how it would be relevant in adding to Construct. I mean, what exactly are you looking for?
Anything I could possibly help with, I would be very happy to help. I would love to get at least a little 3d, even it not the highest quality rendering; so at least you could do topdown 2.5d or something. I think an easy way to go about learning more about 3d rendering is possibly using some other 3d source codes for comparisons? I know there are more than a few open source 3d engines that would probably be of some help.