How to run the code by moving one column to next and repeat the algorithm

I have a code of neuaral network, where my number of input is 3 (matrix dimension is 20 by 3) and target is 20 by 900. I want to run the code with three inputs and 1st column of target and store the predicted output in a matrix A, similar dimension (20 by 900). Then move to 2nd coloumn of target with same input and run the code and store it in A and so on upto 900 array.

I am not understanding how to do it. Please provide me your suggestion to solve it.

Thank you.