Making A input from series data

I have selected the last three load data (Ln−3, Ln−2, Ln−1) as the input variables of the GRNN model, and the output variable is Ln. . Due to using the three load data as the input variables to forecast, the training sample data is started in 1990 and ended in 2005. and the following is function:

(Ln−3, Ln−2, Ln−1) ----> Ln. Example: input of (X1, X2, X3) has output of Y4.

X = [4 6 6 3 8 9 3 4 5 6]

Y = [43 65 34 34 21 54 34 53 29 20]

X1 = X(Ln−3, Ln−2, Ln−1)

Y1 = Ln

s = 0.1

grnn = newgrnn(X1, Y1, s);

Does Anyone how to configure the input including looping in matlab code with requirement above?