My advice to you is
Try to make it simple, one table for all, you will have the Admin, SuperUser and the NormalUser in one table and have a status, lets say if the status is 1 = admin if 2 = SuperUser, and the update per month can be done on that table so you could add a Status_of_update if 1 = yes and 0 = no so you can check if the status = 0 update if 1 = no need to update and you set all the Status_of_update at the beginning of every month to 0 so every one must update. The extra information that is not common for all should be saved in separate tables, this will make the search for username and passwords way faster and much easier to update. This works with most of my projects. Where I only need session_register('status'); so that will be the only thing I need to know if this page is for a user or an admin but of course you need to add some more stuff to make it more secure