For zone 3, 3 intervals 10 minutes each with 2 minute rest between intervals makes most sense. Most training plans I see mean this when they say 3x10. For Macha's interpretation they write 10x3. But usually the books are clearer and say 3x10 min so there's no ambiguity. I assume the training plan will eventually ramp up and prescribe higher intensity intervals which are typically of shorter duration.