Referring Taxonomy terms together

I'm building a car occasion website in Drupal(7).

I have a contenttype "CAR" which has 3 fields:

1. field_car_brand (reference to taxonomy brand), for example: BMW
2. field_car_model (reference to taxonomy model) for example: 3 series
3. field_car_type (reference to taxonomy type) for example: 325i

What i want is that if the user adds a BMW, only the models from BMW show up.

I found the module "Hierarchical Select" , but then i loose the 3 seperate fields, because everything becomes 1 field.

Does anyone know a method to refer everything together without losing the 3 seperate fields?

Thank you in advance.