in japanese visual novel/adventure games, they use cg term in context of event illustrations, where at certain points of the game you're presented a unique big drawn image of a scene, instead of stuff like sprites on a location map or recycled drawings of charaters placed against different backgrounds. if you see cg roles in game credits, they're about these illustrations and not any other art.
in general they also often use cg or 2dcg to mean digital drawings or digitally-colored art.
porting it back into english, with western visual novel devs who aren't from a drawing skills-obsessed culture more frequently using 3d software to render their illustrations, you have the need to differentiate them from 2d stuff, hence 3dcg.