Nintendo confirmed that domestic sales of the shooter crossed 3.45 million units within this period, making it the highest domestic sales for any Nintendo Switch title within the first three days of release.
Splatoon 3 has only been out for three days but is already a major success in Japan.
Nintendo is also working on large-scale paid DLC, though a release date has yet to be confirmed.
There will be new stages, cosmetic catalogues, new weapons and new modes like X Battle and League Battle added in post-launch updates.