Well the phases are pretty vague anyway, not some hard cut off point where 'if you want to understand phase 3, you must watch these movies'. Each movie is building on the setting of the previous ones. It is, not coincidentally more like the set up with comics where there is some lead up to a big crossover event, then some clean up, but it all blends into existing story arcs in the individual books. Rather than say, three separate movies or books of a trilogy where there is a beginning, middle and end not only for the whole thing, but in each part as well.