A trimatrix was a type of Borg subdivision used by the Borg Collective.
A Borg drone named Axum was assigned the designation Five of Twelve, Secondary Adjunct of Trimatrix 942. This trimatrix appeared to be based in the Beta Quadrant. (VOY: "Unimatrix Zero")
It seems that a trimatrix composes one-third of a unimatrix.