If they've been struggling to get even a 3rd film passed Lionsgate for all these years, so I really doubt that they'll create a trilogy of movies about the backstory. Even though it'd be epic if it did happen!
I could see a film with all those stories pushed into one movie though, it could be all put into an hour and a half feature film. Easily.
They should do a prequel trilogy. Robin Weaver and Coffin Rock in 1886. People wouldn't know what hit 'em. Then slam out another movie about Rustin Parr and Kyle Brody in 1941. Then slam dunk the trilogy with the life story of Elly Kedward up until 1786 with an ending that shows what happens to Eileen Treacle at Tappy East Creek in 1825. Green light all three films Lionsgate!