The printer’s manuscript lacks the thus here, while the 1830 edition has it. This verse summarizes information found in verses 12 and 13: “for behold the whole face of the land was changed … and the highways were broken up and the level roads were spoiled and many smooth places became rough”. The use of the thus here in verse 17 is therefore wholly appropriate, even though it is not absolutely necessary. In other words, there doesn’t seem to be a strong motivation for adding the word thus here, especially since the following verse also describes how the earth was broken up (that is, deformed):
And as already noted, the loss of the small words like thus is more likely than the addition of such words (for a complete discussion of this point, see volume 3). Thus the original manuscript probably had the thus here in 3 Nephi 8:17.
When we look at the early transmission of the text, we find that there is one case where Oliver Cowdery omitted thus, although only momentarily:
On the other hand, there is one instance where it appears that John Gilbert, the 1830 typesetter, added a thus:
As explained under 2 Nephi 10:3, the thus is actually inappropriate for that passage and will be removed in the critical text. In that case, Gilbert’s emendation resulted from a misconception of how the text should read.
Summary: Maintain the 1830 reading with the thus in 3 Nephi 8:17 since it is more likely that Oliver Cowdery would have omitted a single word like thus from 𝓟 than the 1830 typesetter would have added it, especially in this context.