Which prefix signifies "three"?

The prefix that signifies "three" is "tri-." This prefix is derived from Latin and is commonly used in various medical terms to denote a quantity of three. For example, in medical terminology, "tricuspid" refers to a heart valve with three cusps, and "triangle" refers to a figure with three sides. The etymological root of "tri-" connects to three distinct concepts or components, making it the appropriate choice for this question.

The other prefixes mentioned have different meanings: "bi-" signifies "two," "tetra-" indicates "four," and "multi-" means "many" or "multiple." Each of these prefixes applies to different contexts and quantities, establishing why "tri-" is uniquely the correct option for indicating three.