Lewis & Short

2. tertĭo, no

  1. I. perf., ātum, 1, v. a. [tertius], to do for the third time (very rare): jugerum, to plough for the third time, Col. 2, 4, 8: campos, id. 2, 4, 4: colles iterandi tertiandique, id. 2, 4, 9: agrum, Pall. Sept. 1, 1: tertiata verba, thrice repeated, i. e. stammered out, App. M. 5, p. 166 fin.
    Hence, for the third time, thrice: verba tertiato et quartato dicere, Cato ap. Serv. Verg. A. 3, 314.
  2. * II. Part.: tertĭātus, a, um, greater by a third: castra, whose length is one third greater than its breadth, Hyg. Grom. p. 8, 1.