How to split data set into three parts?

I have a dataset and I want to split it into 3 parts 1/3 each part, then I apply the same method on other data set which it have different numbers so it will also be divided into 3 parts based on the range of that data set. How can I do that? and is it should be done using Nurul Networks and Artificial Intelligent. My data is 2D matrix from 0-255 (as default), I want to divide into 0-85,86-170-255, but-but-but some matrix does not start from 0 and does not end by 255 that's why I need a method that can calculate for me 3 ranges based on each matrix.