You should just include them in one table and create a field/attribute that would be an indicator of whether the user is an Admin, Supplier or Employee.
It's simpler if you centralize it that way.
The concern on how/what they access would be under the software you develop. You can either fetch/constrict the UI[or whatever they access in the software system] basing from the type of user you have.
I usually just hide and show stuff according to the type of user I have
Hope this helps..
