Category in Combo box

Hi Friends,

Can any one guide me to create this category in combo box. Let me tell you in detail.

First consider this Category model:
Main Category1

Main Category2
——-Sub Category2/1
——-Sub Category2/2

Main Category3
——–Sub Category3/1
——–Sub Category3/2
————–Sub Sub Category3/2/1
————–Sub Sub Category3/2/2

I hope am making sense in this category model. I need to create a combo box for category. So that,

First Combox box should contain Main Categories.
Second Combo box should contain Sub Categories for Main Category.
Third Combo box should contain Sub of Sub Category from Main Category.

For assumption we can take all having equal sub-sub-category for main category.

If I select the main category from combo box 1. The second combo box should automatically select the sub category for that selected main category in combo box 1. similarly if we select 2 comb box automatically third combo box should fill according to input given in the 2nd combo box. Afterward if I select ok. Then should get the exact post in that sub-sub-category.

Please help me to design this kind of structure.

Thanks in advance.

Regards,
M.A. Vinoth Kumar