First of all: Don't worry when you can't find the Triple Crown anywhere, you won't see the tournament before winning the final of either Rallycross, Rally or Landrush. It later gets selectable after winning all of the three (Historic Rally doesn't play any role here).
Like the other achievements for getting licences or winning finals this one is also pretty straight forward. You need to win one huge final event for the three types of racing with a lot of races. Especially the Rally Triple Crown consisting of five locations, each with eight races (two of them at about 15km long). The courses are the most difficult ones and some of them (mainly Wales) with annoying foggy conditions or heavy rain. Focus on driving clean and making no bigger mistakes, since difficulty doesn't matter like for most of the other achievements and restarting because of crashing the car can be time-wasting and definitely also annoying like hell.
Overall you should have it after about 8-10 hours and there is at least a little way to save some time: The Rallycross-events all have 6 races with the first four being qualification-races. When you win two of them you have no problem to reach top 12 to qualify for the semi-final (race 5) despite retiring after the start of the other two qualification-races since you even receive points (28) after those ones and easily finish qaulification around place 3-8. In best case you can skip 10 races of four laps to save 25-35 minutes. Good luck!