In the 1905 LDS edition, the typesetter seems to have unintentionally omitted the the from the phrase “in the which”. In proofing, the error must have been caught, but when the typesetter inserted the the, he accidentally placed it in the preceding line, thus ending up with the anomalous “I am called the Mormon”. It is possible that the handwritten the that had been inserted in the proofsheet looked as if it was inserted before Mormon in the preceding line (which happened to be the first line on the typeset page). The 1911 LDS edition removed the extra the before Mormon, an obvious error, but left the shortened phrase “in which” since this is what we expect in modern English. There is one other place, in Jacob 5:17, where the 1905 edition replaced “in the which” with “in which” (see the discussion under that passage); in that instance, however, the change was first made in the 1902 LDS edition, so there is a possibility that the 1905 reading derives from that earlier edition.
In his editing for the 1837 edition, Joseph Smith made that emendation to “in which” for many of the original instances of “in the which” (see the discussion under 1 Nephi 3:2). Subsequent editions have generally left the remaining instances of “in the which” in the text; there are only a few cases in the textual history where the the in “in the which” has been removed in editions dating after 1837 (such as this one in the 1905 LDS edition for 3 Nephi 5:12). Of course, the critical text will restore the original “in the which” in this and all other instances, providing it is supported by the earliest textual sources. For a complete discussion of the editing of “in the which”, see under in the which in volume 3.
Summary: Restore in 3 Nephi 5:12 the definite article the in the phrase “in the which”; the 1905 typesetter accidentally omitted the the when he first set the type; when he corrected his error as part of his proofing, he accidentally inserted the the in the preceding line, before Mormon (an obvious error that was removed in the 1911 LDS edition).