The printer’s manuscript reads “all that I have caused … ”, whereas the 1830 edition reads “all that which I have caused …”. The original manuscript is not extant here for “all that (which)”, but spacing between extant fragments implies that Oliver Cowdery (at least initially) wrote either all that or all which and that perhaps later he supralinearly inserted the which or the that as a correction (although the possibility remains that he didn’t make any correction at all). In the transcript for 𝓞, I proposed that Oliver Cowdery originally wrote all which and then later inserted the that (see lines 22–23 on page 403ªof 𝓞). Then when Oliver copied the text from 𝓞 into 𝓟, he misinterpreted the that as a correction for which (even though the which was not crossed out) and ended up writing all that in 𝓟. The 1830 compositor, on the other hand, set all that which, the corrected reading in 𝓞. It’s also possible that the which should have been crossed out in 𝓞 but that Oliver had neglected to do so.
The normal expression in the Book of Mormon is simply “all that” (that is, without the following which), but other possibilities also exist. We get the following statistics for relative pronouns in expressions of the form “all ”:
“all that” 32 times
“all who” 7 times
“all which” 4 times
“all that which” 2 times
“all whosoever” 1 time
In other words, the unexpected “all that which” does occur:
One would expect that there would be a stronger tendency to omit the which in an original “all that which” than to add the which to an original “all that”. Nonetheless, it should be kept in mind that the earliest text has 176 instances of that which without a preceding all, as in 1 Nephi 19:23: “wherefore I did read unto them that which was written by the prophet Isaiah”. So there is nothing textually wrong with the fuller “all that which” in 3 Nephi 1:13 and in two other places in the text.
There is considerable evidence that the scribes sometimes omitted the relative pronoun which momentarily. For a list of cases, see under Alma 5:3; that passage is interesting in that it shows the 1830 compositor adding a which, probably because it was expected (see the discussion under that passage). But here in 3 Nephi 1:13, the reading with the which is rather awkward, which means there would have been little motivation in this case for the 1830 compositor to add the which on his own. Moreover, there is virtually no independent evidence for which ever being added to the text, even momentarily. There is one possible interpretation (which I ultimately reject) of a textual variant that assumes an accidental insertion of which; for discussion of that case, see under Ether 14:24. The critical text will accept the 1830 reading here in 3 Nephi 1:13 as the most plausible reading for 𝓞, although apparently as a corrected reading in 𝓞.
Summary: Maintain the 1830 reading for 3 Nephi 1:13: “I will fulfill all that which I have caused to be spoken by the mouth of my holy prophets”; the reading in 𝓟, without the which, appears to be a copy error on the part of Oliver Cowdery.