If I understand correctly your question, you are trying to plot the step response of this function transfer:
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/249164/image.png)
Right?
Then, why not simply using the multiplication of transfer functions if you don't want to do the math?
% transfer function
H1 = tf(500, [1, 8]);
H2 = tf([1, 2], [1, 10]);
H3 = tf([1, 5], [1, 12]);
G = H1 * H2 * H3;
% show figure
figure;
step(G);