Question about working with longitudinal data and z scores

I was hoping someone could help me or point me in the right direction. The outcome variable for my study will be change in math scores over 3 years. I standardized the math score for each year by creating z scores. What I don't understand and can't seem to find in the literature is how to measure change in z scores between the two time points. I can't seem to find anything online (only info on how to calculate z scores). Any help would be appreciated.