You should elaborate on the type of "3D" game you'd like to make. That way people can direct you to different resources.
So yeah, C2 isn't a 3D engines but there are ways to create 3D effects through serious math and coding skills. There's a Doom-like FPS capx on the tutorials section. There are also plugins that can add 3d effects to C2, like the one Gmoney mentioned. There's another 3D platformer plugin that seems pretty cool. Look up 2.5D platformer plugin maybe.
If you want to find capx examples and tutorials, use the search function and type in "3d", "fake 3d", "pseudo 3d" or some other combination of words. I've got some examples I've collected but can't share them till I'm home.