This is easiest if you just create a minimal championship, consisting of three single lap, circuit races using a race with a short track. As the title indicates, you have to win the championship (not necessarily every race). Just make sure that you create a championship requiring European Sports Cars (or at least allowing them, though just requiring makes it easier in selecting a car).
You can stack this challenge together with #Forzathon Take The Lead and #Forzathon Practical Applications. By the time you win the championship you will have easily satisfied all three challenges.
To make it even easier, I have a 3 circuit race championship in the city just northwest of the Byron Bay Horizon Festival that you can use. Each leg of the championship is one lap and all are short tracks. It starts at the East Bridge Scramble, then East Suburbs Circuit or Switchback Scramble, and finally, whichever one is left of the those last two.
For search purposes, my gamertag is Nautius Maximus, just like my TrueAchievements ID. Also (obviously) you should be looking for a European Sport Car championship.
Note that I will be leaving this championship as-is for the duration of this challenge period for any who want to use it rather than create their own. However, this is my "Go to" championship for resolving FH3 challenges because it is designed to be short and easy. I just change the car type to match whatever is required for the challenges O' the week. If you decide to create your own championship for the same reasons and to use similarly, I suggest these same three races. There are others that would do, but these are all short and nearby each other for maximum convenience, creating the optimal minimum effort challenge.