I would get the documentation from my GP and present it after the probationary period, assuming that your training period more or less covers you. I might even wait until I turn down a 3rd shift and present it afterwards. Perhaps this will not happen often enough to worry about?

How can they even do that, though? What about people who have children? Could you just say that you have family obligations that would prevent you from working that shift? I really would wait until it's a definite issue -- this seems like a policy that can't last long... you are not the only one who is going to be unhappy about this.