May 2017 is the 3rd month that the Nintendo Switch has been available for. During the latest month the gap grew in favor of the Switch by 303,882 units. The Switch is currently ahead of the 3DS by 1.48 million 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 4.11 million units, while the 3DS sold 2.64 million units during the same timeframe.