Anstett
Emperor Mongoose
IF you want the complexity of 3D combat and have a VTT it is a good thing. No additional rules needed.Even if the VTTs might do the 3D you would be forcing the table top players to add an additional level of complexity.
No if you want 3D combat ask mongoose for a rule specifically supporting this and leave those modifications to that book
If you want to keep it simpler for those at a tabletop use the same rules.
As pointed out it is simple trigonometry. Anyone could do it now, it just would slow down the tabletop play so (almost) no one actually does it.