Three-part breath is filling the belly, then the ribs, then the upper chest in sequence.
The inhale travels up; the exhale empties in reverse or as one wave. In 📝Breathwork it is a way to feel the whole barrel without a high, stuck chest as the only option. It is a method, not a yoga requirement, not a diagnosis, and not a contest of how full the lungs can get. If the third part hikes the shoulders, it is dropped.
Clients feel a wave from low to high. Dizziness or panic means the inhale is too big — smaller, nasal, maybe only two parts. Chest pain is a stop.