Any race should work, just change the Drivatar difficulty to 'New Racer' to improve your odds. I personally just used the Coober Pedy Scramble race as well over half the time I get a 'Triple Pass' skill almost by accident. Do the exhibition race, blueprint the race to 'Anything Goes', 1 lap, and pick your best car.
*Edit* While going for the #Forzathon End Of The Line challenge I found that the race just north of Coober Pedy Scramble, Cross Track Scramble, is also a really good spot to get a 'Triple Pass' skill.
For reference quickly passing 3 cars in succession *should* qualify for a 'Triple Pass' skill, it may require a few restarts but all in all this took me less than 5 minutes total, restarting about 6-7 times, with load times included.
Wait for a few moments until the cars get a little bit ahead of you, then take off and pass them as shown here. Rewinding can work, but because it is so early on in the race it is easier to just restart and do the same thing.
The first challenge I get is actully the #Forzathon Follow The Leader challenge, for getting 10 'Drafting' skills, so you will most likely randomly get that while going for this.