Here in 3 Nephi 1:15, the original manuscript probably read beheld (that is, without a preceding he), which is clearly an error. If 𝓞 had read he beheld, then the change to behold in the 1830 edition would have involved two changes, which is quite unlikely. Similarly, if 𝓞 had read behold, then the reading of the printer’s manuscript, he beheld, would have involved two changes, another unlikely scenario. It is easier to assume that both the 1830 compositor and Oliver Cowdery (in 𝓟) attempted to emend an obviously incorrect beheld in 𝓞. In the printer’s manuscript, Oliver emended this reading by inserting the subject pronoun he, while in the 1830 edition the compositor changed beheld to behold.
Although either reading (he beheld or behold ) is possible, the 1830 compositor’s emendation, behold, is probably the correct one. First of all, the phrase “for behold” is much more frequent than “for he beheld” (259 versus 4 in the earliest text, not counting 3 Nephi 1:15). Secondly, if “for he beheld” were correct, we would normally expect a following subordinate conjunction that (either after or before the prepositional phrase “at the going down of the sun”). Elsewhere in the text, there are ten instances of “he beheld” followed by a sentence complement, and in each case the subordinate conjunction that is there, as in Alma 32:6: “for he beheld that their afflictions had truly humbled them”.
But most significantly, we have substantial manuscript evidence that Oliver Cowdery quite often accidentally wrote beheld in place of behold, including the following two cases where 𝓞 is extant and incorrectly reads beheld:
For other instances where Oliver momentarily wrote beheld instead of behold in the manuscripts, see the list under Jacob 5:37.
Given all these factors, it is very probable that in the original manuscript Oliver Cowdery incorrectly wrote beheld instead of behold. The critical text will therefore accept the 1830 emendation as the original reading.
Summary: Maintain in 3 Nephi 1:15 the 1830 reading, “for behold at the going down of the sun”; Oliver Cowdery frequently wrote beheld in place of behold in the manuscripts, sometimes without correction; in 3 Nephi 1:15, he apparently made that mistake in 𝓞; when he copied from 𝓞 into 𝓟, Oliver decided to emend the reading by adding the subject pronoun he; the 1830 compositor, on the other hand, correctly emended the beheld in 𝓞 to behold when he set the type.