There were some hints they might be considering this between Miyamoto's software statements and an even older statement from Iwata about increasing the number of SKUs (hardware configurations), but essentially having them share a foundational platform base.
Now, this could mean common APIs that make porting between platforms easier, or it could mean they all use basically the same hardware a la iOS devices (especially last year, where the iPhone 5S, iPad Air, and iPad Mini 2 all used the same processor) and run all the same software despite being different form factors and products you can buy.
You can essentially sell people on the hardware configuration they like the most (or multiple if they want a home and portable version) and then just sell them games that work on any of the devices so you effectively only have to support one platform.