Quantifying relative values of disparity maps

Hello All,
Novice to OpenCV
I have three sets of images(L,R) of the same object taken on three different time points: 1, 2,3 . I now want to quantify the relative differences between the disparity maps of L1R1 and L2R2 and compare it to the difference between L1R1 to L3R3.
Any ideas?
I am using StereoSGBM_create