Why would it be seriously insufficient once GF3 & GF4 come online ?
Looking at the graph in the Q4 ER, it looks like in 2019 about 260-270k M3s were for non-China markets. However, Fremont will also have to make higher trims for China, and I see overall demand for Tesla growing going forward. There might be some cannibalization from MY, but 250k M3 from Fremont does not seem like it'd be quite enough imo.
Giga Berlin is expected to start production mid 2021 if there are no hiccups, but they've stated they will start with Model Y production, so M3 production at Giga Berlin might very well not start until mid 2022. This is two and a half years away. Lower prices due to localized production should also further increase demand in EU at that point.
I also have a feeling that Tesla demand in China is going to be very strong based on the early signs. They may no longer be shipping SR+ to China from Fremont, but I could see Giga Shanghai not being enough to fulfill local demand of SR+ vehicles, and therefore a number of people deciding to order higher trims from the USA.
I could be wrong, and maybe if they manage to squeeze out 12k/week in Fremont, that would be close to fulfilling non-China demand, especially when Giga Berlin starts producing Model Ys and mix at Fremont can shift towards M3. But all in all, I'm not convinced about Fremont production capacity yet, and I'd love some clarification from management.