Splatoon 2 has sold through 670,955 (physical) units in its first 3 days in Japan according to Famitsu. This doesn't include eShop sales. This sales performance makes it the highest selling Switch game in Japan so far and (obviously) the highest selling debut sales of a Switch game in Japan.
102K Nintendo Switches were also sold in Japan in this period. Switch installbase in Japan is now 1,205,087.
The first Splatoon's Japanese debut sales were 156,610 (physical) on a Wii U install base of 2,333,622. Splatoon 1 then went on to sell 1,512,789 units (according to Famitsu) not counting eShop sales.
Source: https://www.famitsu.com/news/201707/26138427.html
Update, Media Create's numbers:
102K Nintendo Switches were also sold in Japan in this period. Switch installbase in Japan is now 1,205,087.
The first Splatoon's Japanese debut sales were 156,610 (physical) on a Wii U install base of 2,333,622. Splatoon 1 then went on to sell 1,512,789 units (according to Famitsu) not counting eShop sales.
Source: https://www.famitsu.com/news/201707/26138427.html
Update, Media Create's numbers:
Media Create report that Splatoon 2 sold 648,085 units and 98,999 Switches were sold (in the same time period).