Yeah, I find this very hard to believe. They'd cannibalize each other; better for Nintendo to space them out by a few months even if Mario is already complete.
I don't know, Nintendo may be willing to launch with Mario/Zelda/Splatoon all on day one just to be able to present the image of "yes, content is coming, and LOTS of it". The single biggest danger to the Switch would be gamers worrying about Wii U style software droughts, so having a crazy strong launch and a packed timeline for the first year will basically cut that argument off at the pass.
Conventional wisdom might suggest that all this software will fight over their different chunks of the pie, but Nintendo is well aware of the fact that their software has sales legs long after the traditional launch window. Nintendo may have decided that dispelling any possible narrative about software support would be worth each of their titles cannibalizing each other a bit at launch, with the expectation that in the long term it would be made up by more people deciding NOT to hold off on buying the Switch.
Nintendo knows, after Wii U, that they need to drive that install base above all else. Install base means both more third party support and more people to sell their software to.