Lewis & Short

trīga, ae, f. [contr. from trijuga].

  1. * I. A team of three horses, or a chariot drawn by three horses, Dig. 21, 1, 38, § 14.
  2. II. A set of three, a triad, three, Arn. 4, 136; cf. Anthol. Lat. 1, p. 439.