I terms of the 3DS, I'd say Nintendo also made the mistake of assuming third parties and developers will blindly trust them with the 3DS because of its predecessor's pedigree, so they decided to cut corners on Launch titles, so that third parties can take the spotlight. But when it became apparent that third parties didn't have much ready either due to them still supporting the DS, the bigger titles not being ready yet, or skepticism of the system versus the PlayStation Vita, Nintendo scrambled to try and clean up the mess they created with their naive assumption. I once heard that Retro Studios was only brought in for Mario Kart 7 just to get the game out in time for the Holiday season.
This mistake would be repeated with the Wii U as Nintendo just assumed the Wii's brand recognition, and increased power would mean the third parties would just crawl to their side en mass. But when the reality of the Wii U became apparent, Nintendo was caught with their pants down, and lost a ton of money on just trying to keep the system afloat as Third parties began packing their bags for the real next gen consoles.
Nintendo really screwed up the 3DS's launch, but they were able to get away with it thanks to Sony making an even bigger mess of the Vita, and Nintendo managing to get a couple of killer apps (Mario Kart 7 and Super Mario 3D Land) out before the Vita even launched.
They were clearly relying on the same thing happening with the Wii U, but this time around Sony absolutely hit it out of the park with the PS4, and even the Xbox One was salvageable once Microsoft ditched Kinect and cut the price. It kinda says everything that even in 2014, when the Wii U hit its peak and the Xbox One was floundering, the One still outsold the Wii U every single week of that year.