It's easy to get drawn into these types of finance arrangements, as you get a new car for a relatively small deposit, and low monthly payments.
A lot of people like the fact that after 3 years you can just hand the car back. So basically you have only financed the deposit, plus the 36 payments, plus interest.
If you want to hand the car back in as a deposit on a new car, then you are locked into the brand (which they want).
If you want to change to a different brand, then you just hand the car back, but you should also have remembered to have saved up the next ~30% deposit for the next car.