3DS had good legs due to decent performance in all 3 regions. The Switch will blast 3DS in the Americas but fall behind in Japan. Let's compare Switch and 3DS, equal quarters.
Switch is 14.01m America, 8.23m Japan, and 12.50m Others. 34.74m Total
3DS was 10.26m America (38.7% of Current sales), 11.54m Japan (45.8%), and 9.29m (39.5%) Others. 31.09m (41.4%) Total.
Switch has a 3.65m lead. 3DS only shipped 12m next FY, while Nintendo predicts 18m for Switch. Clearly Switch will widen the lead.
As you can see at this point in its life (March 31, 2013), the 3DS has sold ~41.4% of its total sales. Its highest proportion of sales was due to frontloaded sales in Japan at 45.8%. I think Japan sales are usually frontloaded though, with the Switch lagging far behind in Japan but far ahead in America and Others. I think we could make a guess and assume some of the % that the Switch has sold so far of its LTD.
For example if we go with 37/40/39. Then we get 37.9m America, 20.5m Japan, and 32.05m Others. Giving 90.45m Total sales. Personally, I think that is a bit strong. At 90.45m it outsells the 3DS by 14m.
3DS is dropping hard in Japan after it's 3rd year (2013: 5.1M, 2014: 3.1M). It's not difficult to imagine that after this year, the Switch will actually start closing the gap to the 3DS in Japan, just like it probably will start to do worldwide against the Wii. I don't expect the Switch to be nearly as frontloaded as the 3DS in Japan.
In fact, the way the sales are going in Japan, it doesn't look like the Switch will loose by much this year already. It is already up by 300k over last year, and that will certainly only grow over the next weeks and months.
The Nintendo eShop rating Thread: http://gamrconnect.vgchartz.com/thread.php?id=237454