There is absolutely no reason that TSLA shouldn't have raised cash with a relatively inexpensive bond issuance or moderate share issuance in the lead-up to the large expenditures required to begin Model 3 production.
Well, apart from the fact that TSLA already has $10b of Debt to pay back over the next few-to-ten years. I believe Tesla did borrow heavily to begin Model 3 production, but then production took longer than expected to get off the ground and Musk decided it was now or never and that they would try to break even with what they had rather than taking on even more debt. At some point they had to demonstrate they could make money, and they've already left it a long time. I'm very glad that it looks like they're going to pull it off.