On steam, I have a single ID. No matter which system I'm on, my games follow me. I get the game on my Windows Desktop and I can then download it and play it on my SteamPC (assuming it's compatible with both systems). Or on my Mac (likewise).
If I own a Wii U and a 3DS, are the consoles joined by a single Nintendo ID? If I buy Super Mario Bros 3 on the Wii U, can I play it on my 3DS?
I know you can pay to upgrade a Wii Console game to a Wii U for a "nominal" fee. But I don't see any mentions about Wii U and 3DS cross compatibility and account inter-twinings.