If you plan to participate at a triathlon race, then you should get used to trainings in a row. You should get used to it in your training phase and combine different training units.
So you get used to race-like conditions, you train under a race-like load and “last but not least” you practice the cloth-changing (that’s not so easy :-). Don’t forget to take additional nutrition with you.
We recommend, to participate in smaller competitions before your main competition to practice the changing of the various sports.