Mercedes Plus3 Where is the catch or think mistakes?

Somewhere I have a mistake of thinking and ask for help with the Enlightenment. I (private customer) would like to buy me a new car i.h. v. about 35k. With disability discount of 15% around 29k. The seller thinks that at the end of the runtime I get a guaranteed residual value of the car i.h. the final rate (17k). Therefore, it makes little sense to pay a lot, right? I thought so at 5k. The monthly rates would be at 240€. Final rate would be around 17k. This 17k I could then either pay, so that I get off the insurance eventually, or take a new one and pay a new one, right? For me that sounds like win win, because then the car gets me cheap in 3 years, if I can pay 17k in the form of the exchange and then by the monthly saving probably could pay the same cash. Yes, the bank earns at the 2.99% interest – from me. However, for me this is a rather good deal monthly, because at the end the residual value is still so high – or see ic h that is wrong and the value of the car would not be at 17k after 3 years? But I don’t think so, because Mercedes also offers the option to sell the car itself and then you would have the money as well (e.g. for a new car). How did I jump? I’m afraid to fall into a loan trap anyway, because I’ve always paid my cars in cash so far and that was done then. Of course I have in mind that after 3 years I don’t want a new car and the 17k would like to pay, so I will make a corresponding monthly allowance – and give it to my parents so that I don’t answer it either. Maybe then in 3 years my life situation has changed already, so I have other obligations and could use the 17k quite well myself, which I have covered monthly. Then I would be forced to get a new car after 3 years already, that is not normal with private customers? Or by now already? I’d like a few thoughts on your part.