At the top of each page, you would want to store the values posted by the previous form into session variables. You would want to check that the post values are set using the isset function and then if they aren't set, then you would want to check if the session variables are set. If neither are set, then you would want to forward back to the previous form, so form 3 > form 2 > form 1. This will prevent jumping to a new form before filling out the previous form and submitting it. You would also have all the form inputs stored in session variables after all 3 forms are filled out.
