You have to have all festivals upgraded to level 5 in order to unlock all of the stunts.
For the Speed Zones you can use any car with good stats to complete them pretty easily.
For all of the Drift Zones I used,
Car: 1974 Toyota Celica GT
Tune: "230+ MPH C-CLASS"
Creator: "Don Joewon Song"
This setup makes the drift zones a lot easier as this car just racks up the drift points!P.S.
You will also unlock the "Underdog" Achievement for getting 3 stars with this car.
As for the Danger Signs and Speed Traps, A fast car can complete most of them without much trouble but, I have created a YouTube playlist of 3 Star PR Stunts that include the more challenging ones, I will include more upon request.
Here's the link: https://www.youtube.com/playlist?list=PLvrkjkOmqksoxaId-KCHk...
I hope this is helpful!