Fluent in three languages

The Collaborative International Dictionary Word definitions in The Collaborative International Dictionary
Trilingual \Tri*lin"gual\, a. [L. trilinguis; tri- (see Tri- ) + lingua tongue, language. See Lingual .] Containing, or consisting of, three languages; expressed in three languages. The much-noted Rosetta stone . . . bears upon its surface a trilingual ...

Wiktionary Word definitions in Wiktionary
a. 1 Able to read or speak three languages. 2 expressed or written in three languages. n. A trilingual person.

Longman Dictionary of Contemporary English Word definitions in Longman Dictionary of Contemporary English
adjective COLLOCATIONS FROM OTHER ENTRIES trilingual (= speaking three languages ) EXAMPLES FROM CORPUS ▪ Varro seems to have stated that in his time Massalia was trilingual , the two foreign languages being of course Latin and Celtic.

WordNet Word definitions in WordNet
adj. using or knowing three languages; "trilingual speakers"

Douglas Harper's Etymology Dictionary Word definitions in Douglas Harper's Etymology Dictionary
"involving three languages," 1834, from tri- + Latin lingua "language," literally "tongue" (see lingual ). Latin trilinguis meant "triple-tongued," and was used of Cerberus.