Here's the fastest and most foolproof way to unlock all three time trial achievementsTime trials are marked on the map by a purple stopwatch icon, and in game by a purple flare.
You need to head to the time trial on the far left of the first map area, in the middle. It's roughly NW of the J shaped lake.
The time trial itself is activated by pressing
on the purple flare next to the telephone.
The time limit here is quite tight, but the vehicle you're given handles very well and the track is short and free of obstacles. I got gold on my first attempt without knowing the racing line, so it shouldn't take more than two shots at it, and the whole thing only lasts 40 seconds.
Just for fun, let me know your winning time in the comments below, and how many attempts it took you
Bear in mind that if you're struggling with driving, you can always change the camera to follow the car. I preferred the default, especially for a short track such as this, but it's pure personal preference