Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
conternans, ntis, P. a., v. conterno fin.
con-terno, āvi, ātum, 1, v. a. [terni], to put three things together, to make threefold (late Lat.), Hyg. Lim. p. 191 Goes.
Hence, P. a.: conternans, ntis, three years old: vitula conternans, Hier. in Isa. 5, 15, 5.