It all depends on the Switch itself. As I said in another thread, the Switch generation will most likely see the comeback of evergreen titles that will keep selling for the whole lifespan of the console.
I think all three titles have a chance to hit 10m but I wouldn't want to make a prediction yet. Zelda is looking good but will Splatoon 2 be exciting and different enough from the original? Will Mario be as high in quality as Galaxy was? We don't know yet. Legs (especially when we're talking real long-term sales) depend on word of mouth and quality. The other two games need to deliver or they'll stop selling after a few months.