KOReader runs on many pocketbook models too ;) Although I am not 100% sure if it works for Lux 3 specifically since I don't have this model to test it myself.
Yeah, I should try KOReader on it at some point. Should work. Problem is just Pocketbook has own proprietary modules to load the touchscreen and display drivers, but that is just kernel space. I am not sure if that is even GPL compliant.