Model X was never part of the original "Secret Plan", which was Roaster -> Model S -> Model 3 (reference:
http://www.teslamotors.com/blog/secret-tesla-motors-master-plan-just-between-you-and-me) Model X seems to be a bit of a side project where the development team took their time getting every little detail to their satisfaction. With Model S selling so well, there was no incentive to hurry.
Model 3 is a different story, because it is the entire point of the "Secret Plan", which is to bring EVs to mass market. Because this product is the overriding goal, and because of competition, I believe that there is much more incentive for Tesla to make sure that Model 3 ships on time.
All the technological pieces are in place. The challenge will be scaling up from tens of thousands to hundreds of thousands of units per year. I think they will be able to delivery by late 2017. The real question is: when will they be producing at 300k/year or greater? This is going to require a Supply Chain genius. Tesla needs a "Tim Cook" if they don't already have one.