In their third generation - i. e. the great grandchildren of the Edomite or Egyptian alien: compare the similar phrase in Exodus 20:5.

The children that are begotten (e) of them shall enter into the congregation of the LORD in their third generation.
(e) If the fathers have renounced their idolatry, and received circumcision.

The children that are begotten of them, Of such as became proselytes:
shall enter into the congregation of the Lord in their third generation; not in the third generation from the time that this law was made, but from the time that any of them should embrace the true religion; their sons were the second generation, and their grandchildren the third; and such might be admitted into the congregation, and be reckoned as of them, both in their civil and church state, and be capable of serving even offices among them, and of marriage with them, as the Targum of Jonathan; which some of the Jews interpret of males only, for females, according to them, might be married immediately as soon as made proselytes, in which way they account for the lawfulness of Solomon's marriage of Pharaoh's daughter (p).
(p) Vid. Kimchi in 1 Reg. 3. 1. or 1 Ki. 3. 1. & Misn. Yebamot, c. 8. sect. 3.

In their third generation - Supposing their grandfather, or great - grandfather turned proselyte, and the children continue in that faith received by such ancestors.