Isaiah 52:6 (King James Bible) therefore my people shall know my name therefore they shall know in that day that I am he that doth speak
The location of the phrase “they shall know” is different in the Book of Mormon text: it is placed after the adverbial phrase “in that day”. The King James Bible has the opposite order, which is somewhat awkward. But the phrase “they shall know” is in italics in the King James text (which means that the Hebrew original lacks the phrase); the subject pronoun and verb phrase are recoverable from the preceding “my people shall know my name”. But this Book of Mormon example is different from other cases involving italics in the King James text in that the italicized portion simply appears in a different place in the Book of Mormon text rather than being deleted or altered in place. The different Book of Mormon word order appears to be intentional; since both 𝓟 and the 1830 edition agree here, 𝓞 undoubtedly read with the same word order.
Summary: Accept in 3 Nephi 20:39 the different location for “they shall know” in the Book of Mormon text, especially since in the King James text these words are in italics.