"To get that $1.4 billion in revenue, they will need to sell 40k Tesla 3s at $35k each, or 3,000 a week. To get it in margin, even assuming the average model 3 will sell for $50k, they will have to sell 12,000 a week or so (at 20% margin)"
It seems like you're assuming that the cash burn will stay at 1.4billion. Expenditure should go down once the production line tooling is complete.
Tesla originally planned to run two production lines but had to drop to a single one, and they're doing it in an expensive and unusual manner, and a second won't fit in their fremont plant, and they still plan to build out the second line in the same unusual 'cold' manner.
This leads me to expect it's reasonable their burn will continue at the same rate.
Furthermore, it's estimated they won't reach 3k a week till the end of 2018.
So, can we assume that if the production has to be multiplied by 10, the cash flow used for operating activities should also be multiplied by almost 10, giving expenses of way more than $1.4bn?
No, because a huge cost for them right now is underutilization of assets (factory, tooling, some workers, etc.). Their utilization rate will improve, and so operating costs should not scale linearly with revenue growth. "Should" being the key word.
"Capital expenditures were $1.1 billion in Q3. The majority of capital expenditures were attributable to Model 3 and Gigafactory 1
production capacity increases. "
It seems like you're assuming that the cash burn will stay at 1.4billion. Expenditure should go down once the production line tooling is complete.