A quick searched reveled that the final payment is due at delivery, or shortly before deilivery [1].
When you finalize configuration, Tesla asks you for a $2500 non refundable deposit (although I have heard that they refund it in some circumstances as a matter of goodwill). This deposit is the same for all models even the Model 3, as stated here [2]:
> And finally when you’re ready to place your order, a deposit of $2,500 is required which isn’t any different than placing an online order for a Model S or Model X.
The only thing I cannot tell you is if the initial deposit counts toward the 2500 deposit or the final price. So Tesla doesn’t get billions in deposits and doing that would probably cause a lot of logistical problems and the media would scream that they are scamming customers and only do that because they are about to go bankrupt.
[1]: https://forums.tesla.com/forum/forums/payment [2]: https://www.teslarati.com/tesla-model-3-design-studio-config...