gpears, I don’t know absolutely that Rose’s German Choc filling can be made three days ahead, but I looked at the ingredients and the cooking method and it seems like you should be OK. Be sure to put plastic wrap against the top to prevent forming a skin, and it will need to be warmed gently when you are ready to use it, just until spreadable. Be sure to take it to 175F like it says in the directions, as that is above the instant-kill temp for salmonella.
The best place to ask this question is over on the blog section, under the heading “Cake Questions Three”. There, it is possible you will get a direct answer from Rose, though she is incredibly busy promoting her new book so it would probably take a few days at least for her to reply.