I don't think the fourth year will be interesting. By then the 3DS will be viewed similarly to how the Wii U has been viewed for the better part of the last two years: Simply not good enough to compete in a worthwhile sales comparison, so other consoles will have to be used as measuring stick.
For comparison, here is the US 3DS sales (in millions)
|FYE 3/31/2011||FYE 3/31/2012||FYE 3/31/2013||FYE 3/31/2014||FYE 3/31/2015||FYE 3/31/2016|
- Mario Kart 7 (12/2011)
- Pokemon X/Y (10/2013)
- NSMB 2 (8/2012)
- Super Mario 3D Land (11/2011)
- Animal Crossing New Leaf (6/2013)
- Smash Bros 3DS (10/2014)
What's interesting about the 3DS is how it's sales trajectory shoots up very early on and then has a slow decay. It was pretty stagnant in the US for the first three years (3/31/2012 was the first full fiscal year). Moreover, a lot of its games were very front loaded with really only Pokemon and Smash in the latter years. Switch is only now just getting Pokemon and Animal Crossing, both games that will do very well. I think if you compare Switch and 3DS's sales and software, the Switch has more potential.
As an aside, I think you can tell the 3DS was propped up. Usually consoles peek in their 3rd year, but in the US it peeked in the first. This was also when it got a massive price drop. It's clear in hindsight Nintendo was propping that system up.
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