3D is considerably harder to do homebrew for than 2D. That's why you're far more likely to see mods for an existing 3D game (like Doom or Quake) than to see 3D homebrew. At least then you can work with a working engine and existing tools for levels and models. Unless you have something like Unity3D or Irrlicht or something similar, writing your own 3D engine is beyond the abilities of most homebrewers. Heck, it's beyond the abilities of most PROS too! Most game devs license an existing engine rather than do their own.