The problem with this type of question is that people perceive "low", "medium", and "ultra" in differing ways.
If u are just talking about resolution, then all three (PC, PS3, and Xbox360) can display full 1080p resolution.
If u are talking about maintaining a decent framerate with high poly meshes and high-res textures, then only the PC can actually achieve this and still keep up a good framerate. (This is why they didn't release the game with hi-res textures in the first place...and then only released the hi-res textures as an addon for the PC.
There are also various other less-impressive techniques which are available to the PC, that are totally omitted on the consoles. Things such as anisotropic filtering, good anti-aliasing, real water and glass reflections (not just cube maps), various subtle lighting effects. real distance rendering, etc.
So in all-around performance, there is no way today's consoles could possibly come close to the image quality of the PC.
For a decent analogy, u might consider what a DVD quality movie looks like compared to a Blu-ray movie...vastly different.