Not that I think Tesla will make such profits in the near future (3 years). But you forgot the profit splitting between the joint venture partner and Tesla. The partner won't be putting up investments for nothing. The batteries still need to be imported, unless M3, MS and MX built in China will use China made LiFePO4 cells.
IMO, the China plan is hastened because of China curbing excess EV production.
With current low volume of production for Tesla, there is very little benefit in setting up another factory in China. The factory cost has to be amortized
over the low volume sales in China. Model 3 will face steep competition in China, where it will need to compete on value, not just status as in the case of Model S&X. I don't expect more than 2k per month M3 sales in China for many years.
China's New Rules: Tesla Must Stay Home, Used Car Factories Go Up In Price