2020 for sure, maybe 2021 at most.

I think most central development on Wii U will wrap after 2016. Ditto for 3DS.

People say Nintendo supports their platforms for 5 years, but they actually don't if you look at it.

GameCube development from their main teams was basically nixed after 2004 and Twilight Princess was moved to a Wii title.

N64 development after 2000 was basically Conker from Rare and that's it (Paper Mario was finished in 2000, NOA just opted to hang onto it until 2001).

Wii development after 2010 was pretty slim too ... Skyward Sword and uh ... Rhythm Heaven were the only titles from their core studio teams (EAD, Retro, EAD Tokyo, etc.). Wii Party was from nd Cube, that's not one of their core teams. 

2019 for 3DS and 2020 for Wii U.

lulz, maybe with no software support. 

You do know that discontinuation means that they no longer produce the hardware, right? The Wii still isn't discontinued lol.

If that's the metric then sure, yeah the original Wii is still going from 2006-2015. 

Well that is the metric if someone asks when something will be discontinued. Nothing else.

My guess:

3DS: 2016/2017

Wii U: 2017/2018

2019 both.

Wii U 2019, 3DS 2020