On the one hand, Musk has been known to exaggerate timelines. All of Tesla's cars have ended up being released behind schedule (the first Model X cars are being delivered tonight, multiple years late). I doubt we'll really have fully autonomous Teslas in three years.

On the other hand, there is one interesting, unique aspect to the approach that Tesla is taking: their cars all have internet access, and could potentially learn from each other. Within a year they're going to have partial autonomous driving on over 100k cars in the wild, which is going to allow Tesla to generate an enormous amount of training data very quickly.

I'm no expert, so I don't really know how valuable this kind of data would be for building new autonomous driving systems, but it seems like it could have the potential to dramatically accelerate their progress in this area.