使用ajax向选择框添加值并组织javascript文件

I want to create a page where everything is loaded via ajax calls, so the URL always stays the same. At first the user will be presented with one box with some choices. According to the choice he makes another one will appear and again according to the choice he makes on the second a third will appear. I have successfully created this logic using ajax bit like the following. I am sorry if the code is a bit meshed up, but I am typing the question from a different machine than my code is located, so please be gentle. If you have any problem I will try to provide as much info as I can. So the htmlI want to create a page where everything is loa