That was the explanation Elon gave via twitter: "Pana cell lines at Giga are only at ~24GWh/yr & have been a constraint on Model 3 output since July [...] Tesla won’t spend money on more capacity until existing lines get closer to 35GWh theoretical." https://twitter.com/elonmusk/status/1117144865299501056

Not sure I understand the difference in costs between 2 lines at full capacity vs 3 lines at 66% capacity. Obviously 3 lines is an extra 50% on hardware cost, but what's the cost of time to refine and make mistakes (not to mention the lost revenue on producing more cars) on the last 33% capacity? Apparently Elon would rather get closer to 100%.

From a cost perspective two lines at 90%+ are better then three at 66%. But considering that Panasonic is putting the brakes on might as well be the result rather than the cause of Tesla's struggles.