Here in Alma 16:3, the 1858 Wright edition supplied a that between “it came to pass” and the following subordinate conjunction before. In accord with the reading of the printer’s manuscript, the 1908 RLDS edition removed this intrusive that from the RLDS text. The problem here in Alma 16:3 is whether the conjunction that might have originally occurred before the conjunction before but was somehow lost in the early transmission of the text (we do not have the original manuscript here). Elsewhere in the text, there are only two occurrences of “it came to pass” followed by a subordinate clause beginning with before, and in each instance that occurs in front of the before:
Generally speaking, we expect the that between “come to pass” and a following subordinate conjunction; even so, there are cases where the that is lacking. For instance, when the subordinate conjunction is after, the that is clearly more common (occurring 67 times in the original text), but there are still cases where the that is not there (6 times). For examples and discussion, see under 2 Nephi 1:1. So with only two other examples of “it came to pass that before <finite clause >”, the one example of “it came to pass before <finite clause>” here in Alma 16:3 should not be considered an error but simply a part of the overall variation in the text. For further discussion, see under THAT in volume 3.
Summary: Maintain in Alma 16:3 the unique instance in the original text of “it came to pass ” (that is, without any intervening that between “it came to pass” and the subordinate conjunction before); variation in the text for other subordinate conjunctions like after allows for the that to be lacking after “come to pass”.