Stage 3 hypertension, also known as severe hypertension, is a classification of high blood pressure in the medical field. Blood pressure is measured using two values: systolic pressure (the higher number) and diastolic pressure (the lower number). Stage 3 hypertension refers to blood pressure readings that consistently fall within a certain range that is considered dangerously high.

The classification of hypertension stages is as follows:

1. Normal: Systolic < 120 mm Hg and Diastolic < 80 mm Hg
2. Elevated: Systolic 120-129 mm Hg and Diastolic < 80 mm Hg
3. Hypertension Stage 1: Systolic 130-139 mm Hg or Diastolic 80-89 mm Hg
4. Hypertension Stage 2: Systolic ≥ 140 mm Hg or Diastolic ≥ 90 mm Hg
5. Hypertensive Crisis: Systolic > 180 mm Hg and/or Diastolic > 120 mm Hg

Stage 3 hypertension falls under the Hypertension Stage 2 category, which means that either the systolic blood pressure is 140 mm Hg or higher, or the diastolic blood pressure is 90 mm Hg or higher.