Matthew 5:43 (King James Bible) thou shalt love thy neighbor and hate thine enemy
Here the 1852 LDS edition changed the original thine enemy to thy enemy. This was probably a typo made by the compositor for that edition, perhaps under the influence of the preceding thy neighbor. The 1920 LDS edition restored the correct thine enemy to the LDS text. The biblical text has, of course, thine enemy since enemy begins with a vowel. In Early Modern English, vowelinitial words took thine while consonant-initial words took thy (just like an versus a in modern English). The Book of Mormon text generally follows this pattern, just as it does with mine and my. For some cases where mine has been changed to my, see under Jacob 5:47 and Helaman 13:5. For discussion regarding thy head versus thine head, see nearby under 3 Nephi 13:17. Here in 3 Nephi 12:43, the critical text will maintain thine enemy, the reading of the earliest text as well as the corresponding Matthew passage.
Summary: Maintain in 3 Nephi 12:43 thine enemy, the earliest reading as well as the corresponding King James reading.