Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
trĭlībris, e, adj. [ter-libra], of three pounds weight, three-pound-: mullus, Hor. S. 2, 2, 33: paterae, scyphi, Gall. ap. Treb. Claud. 17.