Georgia Dome Freestyle (Stadium Events) is the way to go. Left of your start position is a huge ramp surrounded by car wracks. Approach the ramp from the other site (middle of the arena) using your boost (X)* and super jump (Y)* to jump high into the air. Getting 3 stars on air time means that you are close to 3 seconds. This may take a couple of tries.
*You need to upgrade your truck to get the boost and super jump abilities.