Indeed not really a choice
Hey!
I haven´t yet met anyone finishing within the 36 months (including defense) either. What [officially] can happen is that you can arrange with your boss to get paid less (nobody really wants that) and extend a couple months.
Or perhaps the institution where you are is able to give you small extensions via a "stipend" usually the 1350€ (which is already a half time fixed value for phd students), which you do not pay tax, only the health insurance.
Alternatively, you just activate the unemployment fund to simply have some income until end of writing. Technically you shouldn´t work while receiving unemployment fund, but practically i don´t know.
Your boss can´t make you work without a stipend/contract though. That´s the fight you should focus on.
Same happened with me and a colleague here in Hannover. If you need any further tips, please do tell.
Good luck!
A.