It would appear that the editors for the 1920 LDS edition recognized that this verse could be misinterpreted as meaning that Gidgiddoni was also the chief judge, so they changed the conjunction and to as. Of course, the correct interpretation involves ellipsis, as if the text read “and also was the chief judge Lachoneus a great prophet among them”. This kind of construction (conjoining a subject noun phrase at the end of a complete clause) is a Hebraism that is found quite often in the Book of Mormon text, as in the following example:
See the discussion under 1 Nephi 3:28 for further instances of the delayed conjoined subject (for a more general list, see under hebraisms in volume 3). In each case, the critical text will maintain or restore, as the case may be, all original instances of the delayed conjoined subject. In cases of possible confusion, perhaps the best solution would be to place a dash immediately before the delayed conjoined subject:
Summary: Restore in 3 Nephi 3:19 the original and that connected the delayed conjoined subject to the preceding clause: “therefore this Gidgiddoni was a great prophet among them—and also was the chief judge”; there are many examples of this kind of construction in the original (and current) text of the Book of Mormon.