Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
trīmātus, ūs, m. [trimus], the age of three years (post-Aug.): quae (gallinae) trimatum excesserunt, Col. 8, 5, 24: a trimatu, Plin. 8, 43, 68, § 168: in trimatu, id. 8, 50, 76, § 200; 10, 20, 22, § 44; Inscr. Orell. 4713.