I don't think we have a fully 3D mental model. I think we have a set of core invariants that are constant across a wide variety of conditions.
I.e. we can recognise static 2D photos of cats, 2D movies of cats, 3D movies of cats, and real cats in the real world.
The invariants are probably relatively simple - a set of head geometries and head feature shapes/distribution, with some secondary colour and texture confirmation.
What's interesting is that we can recognise modifiers to the invariants - e.g. a shaved cat is still recognisably a cat, but parsed as "cat without fur."
We can also recognise invariants when they're pared down to essentials in cartoons and sketches.
https://www.youtube.com/watch?v=R9qdyXCVNVk
A lot of learning is really just data compression - finding a minimal set of low-resource invariant specifics from a wide range of noisy high-resource inputs.