Who developed the three-stage general adaptation syndrome to describe the physiological responses to eustress and distress?

Answer - Hans Seyle

János Hugo Bruno "Hans" Selye, was an Endocrinologist who postulated, based on physiological and psychobiological analysis, that when an event threatens an organism's well-being (a stressor), it leads to a three-stage bodily response:

Stage 1: Alarm.

Stage 2: Resistance.

Stage 3: Exhaustion.

This model was what he called General Adaptation Syndrome (GAS).