Sir Richard C. Jebb, Commentary on Sophocles: Oedipus Tyrannus

[1062] For the genitive τρίτης μητρὸς without ἐκ, cp. Soph. El. 341 “οὖσαν πατρός,” 366 καλοῦ ι τῆς μητρός. τρίτης μητρὸς τρίδουλος, thrice a slave, sprung from the third （servile） mother: i.e. from a mother, herself a slave, whose mother and grandmother had also been slaves. No commentator, so far as I know, has quoted the passage which best illustrates this: Theopompus fr. 277 （ed. Muller 1. 325） Πυθονίκην ... ἣ Βακχίδος μὲν ἦν δούλη τῆς αὐλητρίδος, ἐκείνη δὲ Σινώπης τῆς Θρᾴττης, ... ὥστε γίνεσθαι μὴ μόνον τρίδουλον ἀλλὰ καὶ τρίπορνον αὐτήν. Dem. 58.17 “εἰ γὰρ ὀφείλοντος αὐτῷ τοῦ πάππου πάλαι ... διὰ τοῦτ᾽ οἰήσεται δεῖν ἀποφεύγειν ὅτι πονηρὸς ἐκ τριγονίας ἐστίν ... ,” “if, his grandfather having formerly been a debtor, ... he shall fancy himself entitled to acquittal because he is a rascal of the third generation.” Eustathius Hom. Od. 15. 42-50 quotes from Hippônax Ἀφέω τοῦτον τὸν ἑπτάδουλον （Bergk fr. 75）, i.e. “seven times a slave. ” For the force of τρι-, cp. also τριγίγας, τρίπρατος （thricesold, —of a slave）, τριπέδων （a slave who has been thrice in fetters）. Note how the reference to the female line of servile descent is contrived to heighten the contrast with the real situation.