I think we are missing a key point here. We don't know for how long Tesla's been working on the Model 3.
It's not like the company can only work on one thing at the time. I'll bet that a team has been working on things like the drive-train, battery pack and probably software for years now.
The stamping tools and metal working parts are surely not done yet. But Tesla could very well have the basic car skeleton all ready to go inside their workshops.
Think about it:
1. Electric motor: Take S motor, simplify, validate, done
2. Batterypack: Take current pack, simplify, agree on size, redesign, validate, done
3. Software: Agree on screen size, agree on S-style canbus protocol, code on simulator, pretty far along
I would also think that they are already in talks with seat-manufacturers, sound-system suppliers and so on.
So I agree with the sentiment that it will take hard work, a bit of luck and maybe a unicorn or two to make it happen. But the fact is that we don't know how long they have been working on the 3.