The Surfers Bridge Speed Trap is in the upper Northeast corner of the map in Surfers Paradise, on the West end of the city, I designate it's position at the beginning of the video. To get 3 stars you need 155 MPH/249.5 KPH.
The Ariel Atom is 108,000 credits from the store if you do not already own it. There are many various tunes to upgrade to X Class, any should work as long as it is X Class, it should cost less than 40,000 credits to put it up to that rank from S2.
I did my approach from the east to the Speed Trap, and got a little over 160 MPH. Even with errors, crashes, and traffic it still took me less than 3 minutes to get.
Video for reference of starting location. As shown it took an extra few seconds for the challenge to pop to don't get discouraged if it doesn't pop right away.