Matthew 5:33 (King James Bible) thou shalt not forswear thyself but shalt perform unto the Lord thine oaths
Once more we see the tendency to replace thou shalt with thou shall. Initially in the printer’s manuscript, Oliver Cowdery wrote shal, which he soon corrected to shalt by inserting the t inline (there is no change in the level of the ink flow for the correction). He made the same scribal error in 𝓟 for shalt later in 3 Nephi 13:5 (where he ended up writing the correct “thou shalt not do as the hypocrites”). But more significantly for this passage, the 1852 LDS edition accidentally changed the second shalt in this verse to shall. Perhaps this error resulted because the shalt here is not immediately preceded by thou as it is earlier in the verse. This typo persisted in the LDS text until the 1920 edition restored the original shalt. For other instances of this error in the text, see under Mosiah 12:11; for two nearby instances, see under 3 Nephi 12:26–27. Of course, the corresponding passages in Matthew 5 have shalt in all these cases.
Summary: Maintain in 3 Nephi 12:33 both cases of shalt, the reading of the earliest text as well as the corresponding King James text.