When the Plan Builder builds out a training plan, it optimizes your schedule around one objective: your A race. In other words, the training that you will complete does not change for B and C races, but rather, they are just worked into your regular schedule.
For B Races, a race opener is added the day before the event, the planned workout is dropped and replaced with your B race, and any workout the day after is dropped to allow you to recover.
For C races, the C race replaces the planned workout on that date, and everything else remains the same.
Sadly, we do not currently have a way to add duathlon or running events into the Calendar as official “Events,” however, this is something that we are looking to develop in the future. In the meantime, your best option will be to add these events as “Other Workouts”, and make the appropriate adjustments to your plan manually based on the logic I’ve outlined above.
Let me know if you have any further questions!