December 2017 is the 10th month that the Nintendo Switch has been available for. During the latest month the gap grew in favor of the 3DS by 206,617 units when compared to the Switch during the same timeframe. The Switch is currently ahead of the 3DS by 879,591 units.
The 3DS launched in February 2011 (however, 3DS sales have been aligned to March 2011, since it only launched at the end of February in Japan), while the Nintendo Switch launched worldwide in March 2017. The Switch has sold 13.36 million units, while the 3DS sold 12.48 million units during the same timeframe.