If there were no 3DS version I'd say it would be easily the Switch version, because it is portable. I still think the Switch version will sell more, but it isn't a certainty like before. Here are my predictions.
3DS : 3 Million
Switch: 1.2 Million
PS4: 1 Million
Total: 5.2 Million
It will sell more than DQIX because it is on three platforms, and some people will double-dip between the versions.