Multiple Dynamic Dropdowns from Database

I am working on a web-based incident tracking system. For the incidents, there will be 3 tiers. A primary "Class", a "sub-class" and a "type." I need three pulldown menus, populated from a MySQL database. First, the client will choose the class, then, based on that selection, sub-classes will appear. Then, based on the sub-class chosen, a list of "types" will appear. What is critical is that these items be queried from the database once when the page is called, and no refreshes are necessary. For this, I'll need some sort of client side script which I am not familiar with. I want the client to be able to add their own Classes, Sub-classes, and Types. Essentially I need a client side script to change the menus once a selection has been made and for all those values to be populated from a server side PHP script. Each tier has its own MySQL table. Classes have unique ID numbers, which should be the value of the form, sub-classes have their own ID numbers and have a CLASS_PARENT_ID which relates to the above tier, and types have their own ID numbers and have a SUB_CLASS_PARENT_ID which likewise relates to the above tier. It doesn't need to look nice or anything. Just three dynamic, database-driven pulldowns, the designing will happen later. I'm sure this is a very simple project, but I may not be explaining it well. Please ask any questions.