tertius

Tertius, tertia, tertium is the ordinal of tres, and the r shifts across in the derivatives: cardinal tres, tria, ordinal tertius, distributive terni, adverb ter, "three times".

Two habits are worth knowing. Latin counts inclusively, so tertio die means on the third day with today counted as the first, which is why "after three days" and "on the third day" describe the same stretch of time in the Gospels and why arguments about it are usually arguments about Latin rather than about theology. And tertius is the standard word for a neutral outsider to a quarrel, as in tertius gaudens, the third party who profits while the other two fight it out.