Micah 5:8 (King James Bible) and the remnant of Jacob shall be among the Gentiles in the midst of many people as a lion among the beasts of the forest
As explained nearby under 3 Nephi 20:16, there has been an occasional replacement in the text of forest with the plural forests. Here in 3 Nephi 21:12, the 1902 LDS edition made the change, but that error was never transferred to any subsequent LDS edition (the 1902 edition never served as a copytext). The earliest Book of Mormon textual sources have the singular forest, as does the corresponding passage in Micah 5:8; thus the critical text will maintain the singular forest here. For a more general discussion regarding the grammatical number for forest(s), see under 1 Nephi 18:25.
Summary: Maintain the singular forest in 3 Nephi 21:12, the earliest Book of Mormon reading as well as the reading in Micah 5:8.