How to save the score on Super Smash Bros Wii U

You can link a single ID to your 3DS and Wii U (this is a fairly recent change). But the games are not "cross buy" in any way similar to the Playstation ecosystem. You can upgrade Wii VC games to Wii U (only if they physically reside on the Wii U console), but buying [GAME] on Wii U, much less Wii, doesn't entitle you to [GAME] on 3DS even if [GAME] is on both. There's also no way to move save data between either system, with case-by-case exceptions (you can do this in Monster Hunter 3 U, but only because of a companion app, it's not built-in functionality like it is on Playstation 3/4/V)

The consoles are joined by ID, but really only for Miiverse, Club Nintendo and eShop balance purposes. This has been a common complaint about Nintendo's ecosystem but there have been no announced plans to change this on the current consoles.

Yes for Mario Kart 8 and Super Mario 3D World. NO for New Super Mario Bros U: in NSMBU, you cannot use the gamepad to control a character when playing multiplayer - you have to use Wiimotes or the Pro controllers only to control characters (the gamepad in multiplayer NSMBU works only in "Boost mode" to give other players boosts). Note that in single player NSMBU you can use the gamepad to control your character.