Will SFML support 3D?
The problem with 3D is that it's so diverse that it's difficult to have a single implementation that pleases most people. That's why there are several existing libraries to choose from (and if you don't like them, you can always make your own or just use OpenGL/Direct3D directly). SFML is more general-purpose, so tying in a largely preference-based library would just turn people away. And even if it were to be implemented in SFML, the 3D support alone would be so incredibly large and complicated that it would overshadow pretty much the rest of SFML.
I wait or start extending SFML for 3D rendering.
If you want to try to add 3D support to SFML, go ahead. However, it doesn't make a whole lot of sense. Most of the API would not "integrate" into SFML (3D isn't as simple as adding a new
window.draw overload), and would mostly have to be separate, so there's little reason for it to not be a standalone graphics engine from the start.