All movies are AU stories and not meant to be treated canonically. The same can be said of Naruto, Dragonball Z, and any other shonen action series. They're similar to anime filler, like the Squad 3 Captain arc or the Zanpakuto Rebellion arc.
In terms of theoretical placement, Memories of Nobody would probably be just after Ichigo and his friends return to the living world after rescuing Rukia, though Rukia is with them--he clearly doesn't have Visored powers. DiamondDust Rebellion and Fade to Black both probably take place just prior to Ichigo and company venturing into Hueco Mundo to rescue Orihime.