Hi!
I'll start with the simplest question: the babbling would count as the 1 voice actor.
Regarding whether 3D would be viable: yes, but using the 3D modeling as a tool to achieve a "flat" result.
For the background, what you'd have to do is build your models and set up the scene in whatever software you'd use (Blender, etc.), and then export it as a single 2D render. Many VN devs actually use this process as part of their workflow (here's an example) so as long as the 3D scene is actually not in the game, that'd be okay!
Same thing with the character sprite: model them and then export as a .png or whatever.
Hope that helps!