Finish all Crash Mode stages with Golds . Hold White and Black while selecting a car at the car selection menu. The regular cars on the road will be replaced by the ones in Need For Speed Underground. A few vehicles like the bus won't change.

Progressing in Burnout 3 involves accumulating nothing but Golds, accumulating dozens of takedowns, points and damage money. So make sure you keep those stats stacking and stacking.