Two Dropdownlist - three tables

I have Three tables:
Category
  - CategoryID=1, Category= Vehicles
  - CategoryId=2, Category = Electronic

SubCategory
  - SubcategoryId=100, CategoryID=1, SubCategory=Car
  ...
  ....
  - SubcategoryId=200, CategoryID=2, SubCategory=Cumputure
 
SubCategoryTypes
- SubcategoryTypeId=2000, SubCategoryID=200, SubCategoryType=CD

What I want to do is a search function. I want to fill 2 dropdownlists with data.
DropDownList 1 should contain:
All categories 
-- VEHICLES -- (this should be colord, maybee a css style)
Cars
trucks
...
...
-- ELECTRONICS -- (this should be colord, maybee a css style)
Computures
...
:::
This is a sample what the dropdownlist should contain. 
But let us say If I choose Computures. Because Computurees has a Types also(SubCategoryType) then a new dropdown should bee visible on t ...