Measuring improvement in 3D tasks
How do [I] design [a] test for measurement of [3D rotation] improvement?


The way to start is to define what you wish to measure. 


What needs to be mentally rotated? 
Why does it need mental rotation? 
What goal needs to be reached by the task?
Are the tasks a matter of correct or incorrect, or is it a graded measure?


Only then can you start thinking about the question proper, namely how performance can be measured.