as far as i see, c3 is just an archiecture guideline. They recommend to implement it by using an Ioc framework.
If you use the Messaging function of the Parsley framework for example , so sure you can just call the functions an the views directly to establish communictaion between the components. But you porbably will act not according to the architecture guidelines.
Instead of calling the view functions directly you may prefer to call the functions on the PresentationModels of views and making the views update them selves. If a complex update logic on model cahnge is required, the Observer class of c3 library may be helpfull.
Read more about it here
http://forums.adobe.com/thread/590272?tstart=0