When the first month sales report for the Nintendo Switch report came out, it was obvious that it was going to be a continued success, and it really wasnt a question of if it would outsell the Wii U, but when that would happen, and it just happened. So now I feel like it is only obvious to assume that at some point the Switch will surpass the sales of the Xbox One and 3DS since both have been slowing down in sales.
My prediction - The Switch has sold just under 15 million units, so for simplicity lets say the Switch will sell 16 million for its first year. Assuming Nintendo can keep production up and continue to deliver a library of great games, they should have no problem selling 20 million units per year. So I believe by late 2019 the Switch will be caught up the the Xbox One, and by the end of 2020 (assuming we dont get a Switch 2 by then) it will surpass the 3DS.