The epistle in 3 Nephi 3:2–10 is from a single person, Giddianhi, so the 1830 reading in the plural, “the leaders and the governor of this band of robbers”, is very likely an error. The 1837 edition restored the singular reading. The use of the singular leader is confirmed in 3 Nephi 4:17, where the text refers (at the death of Giddianhi) to the choosing of a new leader for the band of robbers:
Further, in conjuncts like the one here in 3 Nephi 3:1, the singular leader is always conjoined with a singular noun, but the plural leaders is always conjoined with a plural noun:
conjuncts of singulars
conjuncts of plurals
Thus all the internal evidence indicates that in 3 Nephi 3:1 the original text read “the leader and the governor of this band of robbers”. And we also know that in the history of the text, there has been a strong tendency to add the plural s as well as to drop it, so it is quite possible here in 3 Nephi 3:1 that the 1830 compositor accidentally set the plural leaders. Another example of such a typo, one that is obviously wrong, was made by the compositor for the 1841 British edition:
Summary: Maintain the 1837 restoration of the singular leader in 3 Nephi 3:1 (“the leader and the governor of this band of robbers”), the reading in 𝓟.