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
There has been a tendency in the history of the text to replace the singular forest with the plural forests. Here in 3 Nephi 20:16, the 1953 RLDS edition made this change; that edition is known for changing singulars to plurals (see the list of examples under 2 Nephi 7:3). The singular is correct since the as-clause is a literal quote from Micah 5:8, which reads in the singular in the King James Bible. For the same change to forests, but in the 1902 LDS missionary edition, see nearby under 3 Nephi 21:12.
Summary: Maintain the singular forest in 3 Nephi 20:16, the earliest reading as well as the reading in the corresponding passage from Micah 5:8.