I think release dates will be a key factor and whether or not those franchises will be to reach former or new heights ...

I still think Splatoon 2 has the highest chance out of them all since it is already released and is selling in significant enough quantities ...

Quite a few listed might not reach 3M on Switch in their lifetime such as SMO, MK8D, YW4, SSB, MH and especially DQXI ... 

I'm thinking either Spla2n via reaching it eventually, Pokemon or Animal Crossing. With AC more likely to release sooner than pokemon, I'll go with AC.

Splatoon 2... and it will happen in Q3.

Splatoon will get there. Pokemon... well, I'm scared to see the numbers it will hit.

DQ XI on Switch won't hit 3 million at all in Japan. It's already been released on the 3DS/PS4 and the Switch version will likely just be the PS4 version toned down.

To answer OP's question, Splatoon 2 is the obvious answer, as long as AC doesn't release this year.

Lol DQ 11. That game will be lucky to hit 1m in Japan. And only splatoon 2, pokemon and maybe animal crossing can sell 3m

I'm inclined to agree with this.  You can't expect much from a 2+ year old port unless Square Enix adds a lot of content to it.

Splatoon 2. No doubt.
But I expect other games to surpass that milestone: SSB, AC, Pokemon and maybe a new 2D Mario.

Unless pokemon launches this year, Splatoon 2.

I'm guessing Mario Kart 8 Deluxe here. I think it'll keep up a nice attach rate, whereas Splatoon 2 might thin out somewhat more quickly.