The Design exam of VCDX3 was probably the weak part of the entire path.
The suggested course does not cover the exam at all (but to be onest was useful for the application part).
The exam itself maybe was not completly oriented on the design aspect, but there were also some questions about administration, installation, and so on...
If found it also not so difficult to pass.
Anyway the exam is no more avaible, and the new one (for VCAP-DCD and VCDX4) seems a lot better: