3DS is a very different case, as that's a completely different form factor. Even despite that, some info leaked a while that indicated more games were planned for the 3DS beyond what we got, but got cancelled due to poor sales post-Switch.
The lines between console generations have blurred significantly. The similarities between this hardware generation and the last are close enough that different versions of games aren't necessarily specifically developed, it's more like optimizing graphics between varying power capabilities of PCs.
Nintendo went into the Wii U with full confidence that most of the Wii install base would buy into it. I don't think they're going to be as quick to close off the Switch install base, as success isn't entirely guaranteed.
Obviously major releases like a new 3D Mario, Mario Kart, or a new Zelda will be exclusive, but smaller stuff like Mario Party or a Mario Sports title, I could see being multi-platform, at least until their next system is a guaranteed hit. I don't think 3rd parties will be quick to drop it either.