The 1920 LDS edition removed the perfect auxiliary have from 3 Nephi 15:1. This editing is consistent with standard English, which avoids the perfective when the larger clause has a specific adverb of time (in this case, the subordinate clause “before I ascended to my Father”). It is possible that the have here is an error that resulted from the earlier use of the perfect have in the verse (“ye have heard the things”). The 1920 change was intentional since it is marked in the committee’s copy.
There are also two places in the text where the editors for the 1920 edition added the perfect auxiliary have (in distinction to its deletion here); for those examples, see under Alma 1:1 and Alma 31:8–9. More generally, there has been considerable variation in the use of the perfect in the history of the Book of Mormon text; some additional examples of this variation are listed under Alma 31:8–9. As explained there, for each of these cases involving the perfect auxiliary the critical text will follow the earliest reading, even if it seems strange to modern English readers. Thus here in 3 Nephi 15:1 the original reading, “which I have taught”, will be restored.
Summary: Restore in 3 Nephi 15:1 the perfect have, the earlier reading (“the things which I have taught before I ascended to my Father”); the Book of Mormon text allows for more variation in the use of the perfect than what is expected in modern English.