The goal is producing 60,000 Model S and 60,000 Model X a year on two production lines. That's 1154 a week based on a 52 week year. If they plan to be shut down to two weeks a year, that's 1200 a week. The Model X though is a beast to produce and I doubt it's production numbers will ever be the same as the Model S unless they make some design changes to make it easier to manufacture.
Both lines now are running at pretty close to full, compensating for the slower build rate for the X. They did tool the Model S and X lines so they could build the other car, but the two cars share the same platform. The Model 3 will be a completely different platform and tooling one or both of the S/X lines to make Model 3s too would be more trouble than its worth.
I believe in its heyday the NUMMI plant ran 5 lines to produce 500,000 cars a year (each line producing 2000 cars a week). Tesla would be better served maxing out the factory and building out three Model 3 lines. Even with three lines building 2000 cars a week, it will still take Tesla 5 quarters to build 400,000 Model 3s.
I would no be surprised if Tesla wasn't quietly talking to some other countries about building a new plant and battery factory there. They had planned all along to build cars in more places than just the US, but the unexpected popularity of the Model 3 may be advancing those plans. I'm sure many European customers would love to see Teslas built in Europe, if for nothing else to shorten the wait to get a car.
