(immunology) The three cardinal circulatory responses of the skin (i.e. reddening, flare formation or flushing of adjacent skin, and wheal formation or swelling) to any form of injury, such as pricks, scratches, burns, exposure to corrosive chemical agents, and so on.
(botany) The three characteristic morphologic changes on an etiolated seedling in response to the presence of ethylene. These morphologic changes include inhibited root elongation, enhanced radial swelling and shortening of hypocotyl, and exaggerated curved apical hook.
In immunology, the three typical reactions of the skin to injury includes: (1) formation of erythema or red spots caused by capillary dilation from endogenous histamine like molecules released at the site of injury, (2) flare formation or expanding of reddening on adjacent skin caused by neighboring arteriolar dilatation, and (3) formation of wheals caused by increased vascular permeability and fluid accumulating at the site of injury.