For the 1837 edition, Joseph Smith made two editorial changes here in 3 Nephi 5:1. He deleted the word thing, reducing “in the least thing” to the more normal English expression “in the least”; he also deleted the word in that immediately precedes the words. Since both 𝓟 (prior to Joseph’s editing) and the 1830 edition read “which did doubt in the least thing in the words of all the holy prophets”, 𝓞 probably read this way as well (for this part of the text, both 𝓟 and the 1830 edition are firsthand copies of 𝓞).
Although “in the least” (with no noun following) is common enough in modern English, the original Book of Mormon text has only one example of it:
The King James Bible also has a single example: “and he that is unjust in the least is unjust also in much”(Luke 16:10). In both of these cases,“in the least” is semantically equivalent to “in the least thing”.
Yet there is one other case in the Book of Mormon where “in the least” precedes a noun:
In this example Joseph Smith did not edit out the word point, although he could have without making any difference in meaning.
Another example of this usage with a noun can be found in a revelation given through Joseph Smith to Martin Harris in March 1830 (about a year after the time that Joseph had been translating the Book of Mormon):
(A revised version is found in Doctrine and Covenants 19:20 but still with the phrase “in the least degree”.) Thus there is nothing wrong with the more specific expression “in the least ”. In his editing of 3 Nephi 5:1, Joseph wanted to simplify the entire construction, thus he deleted the thing as well as the in. Perhaps he deleted thing because it seemed too general or perhaps because he wanted to avoid two nouns phrases in a row (“the least thing the words”).
The case for restoring the preposition in after the phrase “in the least thing” is more problematic. Normally, in modern English the verb doubt takes a direct object as its complement, not a prepositional phrase, but the Oxford English Dictionary cites examples in earlier English where the verb doubt took a prepositional phrase as its complement. The OED records two early examples where the preposition was in, one in Middle English and the other in Early Modern English (original spellings retained here):
In modern English, when we have a noun doubt, the occurrence of a following in is perfectly acceptable. For example, we can have a sentence like “he expressed no doubt in his brother’s word”. But when we use doubt as a verb, the use of in is unexpected if not unacceptable. For instance, “he doubted in his brother’s word” seems strange compared to the normal “he doubted his brother’s word”. Still, the use of the in with the verb doubt is understandable since in can follow the noun doubt. Quite possibly, the earliest text’s use of in as part of the complement for the verb doubt was allowed because of the intervening phrase “in the least thing”. Of course, one could argue, contrariwise, that this in was an error prompted by the in at the head of the phrase “in the least thing”. The critical text will restore the in since its occurrence here isn’t that objectionable and it may have been fully intended in the original text, especially given its use in Early Modern English.
Summary: Restore in 3 Nephi 5:1 the noun thing as well as the following preposition in; Early Modern English permitted the expression “to doubt in something”, so its occurrence in this passage may have been fully intended.