I'm looking at buying Splatoon, Mario Kart 8 and Smash Bros Wii U but would it still be worth it to do so if I plan on buying the Switch?

We don't what the Switch versions of those games will be (remasters? ports? sequels?) or more critically when they will even be available on Switch. And won't until at least January,

So if you want to play them before then, or even in Q1 2017, I'd say yes gte the WiiU versions. If you want to play them in the latter half of 2017, I'd say wait.

I'd look into whether Splatoon still has an active (and newcomer friendly) community before buying it now. I'm sure those other games still have loads of people playing them. Otherwise, if you already have a Wii U I don't see any reason to wait for Switch versions. All of those games run fine on the Wii U.

They're all good games - if you own a Wii U and don't have them, they're worth picking up.

That said, I'm sure we'll see versions of all three on the Switch, so you could wait if you're not too desperate. But they're all good enough to justify the purchase IMO, even if you end up buying their sequels on the Switch at some point down the line.