Is it ensure, insure, or assure?
early 15c., from Late Latin ternarius "consisting of three," from terni "three by three," from ter "thrice," which is related to tres "three" (see three).
ternary ter·na·ry (tûr'nə-rē)adj. Composed of three or arranged in threes, as a chemical compound containing three elements. n. A group of three.