Matthew 5:36 (King James Bible) neither shalt thou swear by thy head
The printer’s manuscript reads thy head, in agreement with the King James reading. The 1830 edition, on the other hand, reads the head. There is considerable evidence in the transmission of the text that thy can be replaced by the, although the other examples appear to be restricted to the manuscripts; for examples, see under 2 Nephi 20:30. The 1908 RLDS edition and the 1981 LDS edition restored the determiner thy to this passage. The critical text will also follow the reading in 𝓟.
Summary: Maintain in 3 Nephi 12:36 the possessive pronoun thy (“by thy head”), the reading in 𝓟; this reading agrees with the King James reading; the 1830 compositor seems to have accidentally replaced the original thy with the as he set the type for that edition.