Don't get me started, 99% of people here, including mods can't tag games correctly at all.
2DCG is 2D preprendred, 3DCG is 3D prerendered, prerendered meaning it was rendered out by a 2D/3D capable engine into images or video, 2D game would be realtime 2D graphics (for example a 2D unity/unreal game where renderes are done by engine in real time, or rpg maker game, but not renpy, renpy can't render graphics in real time, at least not well enough to be used seriously for 2D application, 3D is not possible at all, it's a joke engine for crappy VN's (mostly), hence frequent use of 2DCG and 3DCG) and 3D game is realtime 3D game like any other 3D game you've ever played (with exception for old games like Wolfenstein or Doom which aren't actually 3D games).