Hi Martijn,
that could not be done using a Data Flow, since a Data Flow that starts from a component inside a Page can only propagate data to other components of the same Page or to an Action outside the page.
In your case the Data Flow starts from page A and goes to Action C, but Action C can only be reached from page B.
As solution, I suggest you to keep the data you want to pass to the Action C also on page B. Or in alternative you can model everything inside one page and access the Action from this page.
Regards,
Lucio