Most people consider their 3D character art as Concept Art. However, this is not the case with the contest. Concept Art is targeted towards 2D artists and illustrators. So, if your character art is for games, enter Game Development. If it's for blockbuster films, enter it in Visual Effects and so on.
3D character artists can enter the Concept Art category, but make sure your 3D work isn't the primary focus of the art. It should be a supporting element eg: 'I use 3d to help block out perspective or help pose my characters and even add a few complex 3d details and that's it, the rest is 2D."
The key here is that your work should not be 100% rendered if you want to enter Concept Art - this rendered work needs to be over in industry-specific categories like Game Development, Visual Effects, 3D Animation.