To obtain three new cars, go to the game's directory and look for files called Pcar1.raw, Pcar2.raw, and Pcar3.raw. Rename they as the name you like. Then look for files called Pcar21.raw, Pcar22.raw, and Pcar23.raw. Rename they as the first three files you changed the name, and the first files, as the second files you changed name. Then launch destruction derby, go to the car selection and you can select three new cars!
NOTE:Your new cars will drive exactly as the others, because you changed the directory game uses for load car body designs. (You trick the game). To drive new cars as their own handling, you must complete the game.