3 regular display boxes is 72 packs, but 6 are URs, so you'd get 66 rares. With 72 rares in the set, it's impossible to complete the set with just 3 regular display boxes.
2 super display boxes is 48 packs with 2 rare slots each and 4 URs, so you'd get 92 rares (96 minus the 4 URs). So there's a decent chance to get all of the Rares in 2 super display boxes.
@Crashfach - Unless I missed something, I think the super packs have 8 commons per pack which would come out to 192 commons in a box.