As I understand your question, you're asking if this part can be used in a BMW X3?
I doubt it, this part was made specific for the Mazda 3 dash. It is a direct replacement for the factory dash piece.
Seeing as the X3 does have an in-dash option I would assume that somewhere out there you can indeed purchase either the housing for the factory nav and just add your screen OR you can find an aftermarket version that fits in the same spot.
Here is a picture I found of an X3, is this what you're talking about?
After a bit of research I do know that a 2-DIN unit, for example
http://www.autoaccessorystore.com/ho...detail&id=1063 would fit. Note that this is NOT a carPC monitor. It is a in-dash radio unit with a touch screen that controls the unit. No USB-touch screen kit could be added to control the PC.
I do know that after-market kits exist to allow you to add video output to your factory navigation screen (see
here) but it would require the factory navigation and I don't know much about your paticular model.