USB3 Vision defines a communication interface for video applications based on the USB 3.0. standard. It could be thought of as an alternative to the already existing and widely used USB Video Class (UVC) specification. It defines the application level protocol used for communication between application firmware on the device and the host driver/application. This protocol is built upon the USB 3.0 physical and protocol layers without modifying anything on USB 3.0 as such. There is no standard host driver defined for this standard (unlike UVC).
Both UVC and U3V are supposed to support RGB format. Yes, U3V firmware can be created for FX3 following the U3V spec. We can provide you an example with U3V on the USB side, but it is not complete and you yourself need to take it forward. Our example can be used as a base for your developments, you can create a tech support case for us to share the example.