Stage 3 is where the Fiesta ST stops being a sports car and turns into a race car. Stage 3 means that the car has everything a stage 2 ST has plus a turbo upgrade. Large turbos can significantly boost the hp and torque of the car while not significantly impacting the weight. The car can end up pushing over 300 hp and over 350 lb-ft of torque.
Stage 3 cars are known for requiring significant maintenance. Many stage 3 cars require maintenance before every track day, sometimes requiring hours. These cars are fast and fun but become pricey quickly as the mods need upkeep, it is not simply the investment in the turbo.