Splatoon 2 had the advantage of being on the Switch in first the place though, contrary to a console that barely anyone bought, so I don't think the addition of Salmon run really impacted a whole lot. Heck, Mario Kart 8 deluxe isn't even a new game and it's the best-selling one on the Switch. Hopefully 3 can entice the veterans.It depends on whether there's a new easily accessible mode from release. Salmon Run proved to be a great addition to Splatoon 2 since it gave a major difference between it and the first game. While Splatoon 3 has shown Salmon Run, it has yet to show any new mode with enough differences from Splatoon 2.
All of the previous game's weapons are confirmed to return, so there's that going for it. Honestly, it just needs to keep the veterans coming back for it to achieve a good sales level, because people who haven't played it yet will almost certainly get Splatoon 3 to see what it's all about.
I think it's more likely the Switch just has a bigger pool of people completely new to the game. 107 million vs 13 million units.Mario Kart 8 Deluxe fixed the otherwise annoying Battle Mode from the original Mario Kart 8, it thus became new enough to get the sales.