Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
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.