Here the printer’s manuscript reads “yea & in fine”, while the 1830 edition is missing the and (“yea in fine”). Each is a firsthand copy of the original manuscript. Spacing between extant fragments of 𝓞 is so large here that we cannot tell if Oliver Cowdery wrote an ampsersand in 𝓞 or not. The 1908 RLDS edition did not restore the and here, even though 𝓟 has it.
Elsewhere in the text there are six occurrences of “yea and in fine” and three of “yea in fine”, so either reading is possible. None of these other examples show textual variation for any of the words in the phrase. But when we look at the much more common phrase “yea and”, we find a rather frequent tendency for the and to be dropped after yea. Interestingly, most of the examples are from the printed editions, including one from the 1830 edition (marked below with an asterisk):
On the other hand, the instances where and has been added are fewer, but there is one that Oliver Cowdery himself added to the text (marked below with an asterisk):
Comparing Oliver Cowdery’s errors against the 1830 compositor’s, we find evidence for Oliver adding and after yea (in Alma 38:7) and the 1830 compositor omitting it after yea (in Jacob 2:6). More generally, in the copying process there has been a stronger tendency to omit small words rather than to add them. For a preliminary discussion of this issue, see pages 121–125 of my article “Critical Methodology and the Text of the Book of Mormon”, Review of Books on the Book of Mormon 6/1 (1994): 121–144. For the complete discussion, see volume 3 of the critical text. This tendency is clear with respect to the phrase yea and, for which there are 11 instances of omission of the and and 4 of addition. Here in 3 Nephi 1:17 the odds favor the reading in 𝓟 over the 1830 reading; the critical text will therefore restore the and to this passage.
Summary: Restore the and of the printer’s manuscript in 3 Nephi 1:17 (“yea and in fine”); the 1830 compositor appears to have accidentally skipped over the ampersand as he set the type from 𝓞.