Matthew 6:34 (King James Bible) sufficient unto the day is the evil thereof
The Book of Mormon text switches the positions of unto and is, which makes for a more acceptable English syntax (although neither expression is especially felicitous). Both seem to be saying about the same thing, although the King James Bible does accurately translate the Greek (essentially word for word). The Book of Mormon variant in word order seems to be intentional rather than accidental.
Don Brugger has pointed out (personal communication) that there is some evidence from for the parallel expression “sufficient is the day for the evil thereof ”, at least from the early 1700s on, with examples from William Penn (died 1718) and William Anderson Cawthorne (1846). Of course, the preposition in this expression is for rather than unto or to. But examples like these show that the Book of Mormon reading is possible.
Summary: Maintain in 3 Nephi 13:34 the switch in the placement of unto and is since it appears to be motivated by an attempt to clarify the original King James word order.