MS/Panos seem like they're winging it on their mobile hardware releases, like this stuff isn't planned in advance. If it was, I'd think the Surface Pro tablet would be the core/base unit they'd produce, and then the 'Surface Book' would just be the hardcore laptop-style base, with the extra battery and GPU in it, to attach the Surface Pro tablet to, and the 'Surface Laptop' would just be a basic laptop-style base for it, lighter and thinner with no battery or GPU, while still allowing the detaching of the Surface Pro for tablet usage. The Type Cover keyboard would still be available for ultra-portability.
It seems weird to have them as three separate products, competing with themselves, when they could all be modules allowing for customization on a whim.