Do I need to register for the Trifecta?

No, you don't need to register to validate the Trifecta. All you need is to finish the 3 race formats (Sprint, Super and Beast) anywhere in the world and you will automatically validate your Trifecta!
At the end of each race, you will receive a piece of your Trifecta medal so after crossing the finish line of the 3 races, you will have completed your Trifecta medal!