Mofo, I know, it's such a puzzle, maze, which is exactly the point.
I thought that, too about it being limited to the 3 levels, but I think somewhere it mentions that more is unpredictable but not impossible.
Because Ariadne only designed three levels and because it was Mal (Cobb's projection) that shot Fischer they would need to find her to find him. They would have to go into Cobb's dream and his dreams are consumed by the world with her, so that level projects that world (their Limbo). If they had gone into Ariadne's mind she would have no clue of where to go as she had never experienced it. It could be Fischer's dream making it easier to find him, but I don't think Fischer would have dreamt of Cobb and Mal's place, so I think it's Cobb's mind, his dreams of the world they built but they are not in a state of Limbo.
Remember how Cobb keeps telling the others not to do certain things, don't bring real memories into the dream, make up new stuff, etc.? Yet, he constantly does what he tells them not to do.
Ariadne and Fischer are able to ride the "kicks" back up which I don't think you can do from Limbo. If they both died from the fall under the heavy sedation they would have gone to Limbo instead of the falling sensation being a "kick" to push them back to the fortress. So, if it wasn't Limbo for them it couldn't be for Cobb, but he stayed behind missing the kicks. He couldn't wake up in the van when Arthur tried to stir him, so Arthur knew something happened. Does that make sense, if it's even possible to make sense out of this thing?
Not that I need one, but it's a good excuse to watch my DVD again.