There are two minor variants in this verse. First, in the 1858 Wright edition the subordinate conjunction if was omitted, probably accidentally. The 1874 RLDS edition followed the reading of the 1840 edition with the if. As explained under Alma 36:7, either reading, with or without the if, is textually possible. The critical text will maintain the if here.
The second variant was introduced in the 1920 LDS edition, namely, the addition of the preposition by (thus changing “as if it were fire” to “as if it were by fire”. Such an emendation is consistent with usage elsewhere in the text:
The second example is quite similar to the example here in 3 Nephi 19:14 in that both refer to being “encircled about”. Ultimately, there is nothing particularly wrong with the earliest text in 3 Nephi 19:14 (“they were encircled about as if it were fire”); in initially reading the sentence, one could misinterpret the pronoun it as referring to a specific object. Maybe that is why the 1920 editors decided to add the by (the addition was clearly intentional since it was marked in the committee copy). The critical text will restore the original reading since there is nothing particularly wrong with it.
One could propose that the original text actually read like the 1920 text and that somehow the preposition by was accidentally omitted during the dictation of the text. Although this is possible, there is no specific evidence for such an error in the history of the text. In contexts where by is syntactically optional, there are no examples of the loss of by except when that by is a repeated by in a conjunctive prepositional phrase; for discussion of those examples, see under Alma 2:38.
Summary: Restore in 3 Nephi 19:14 the earliest reading without the preposition by since there is nothing especially difficult about “they were encircled about as if it were fire”; also maintain the subordinate conjunction if in this sentence since it occurs in the earliest text.