There is a small extant fragment here of the original manuscript, and it definitely has these, not those. The printer’s manuscript has the same these, but the 1830 typesetter accidentally changed these to those. In accord with the reading in 𝓟, the 1908 RLDS edition restored these, but the LDS text has retained the 1830 those.
This kind of error is, as we have seen, very common in the history of the Book of Mormon text. The original these should be restored. There are other cases of variation between these and those in this part of the text where both 𝓟 and the 1830 edition are firsthand copies of 𝓞 but for which 𝓞 is not extant. In those cases, we must rely (where possible) on errors in transmission and internal evidence (usage elsewhere in the text) in order to determine the reading in 𝓞. For some examples of this indeterminacy, where 𝓞 is not extant but 𝓟 and the 1830 edition differ with respect to these and those, see under 3 Nephi 1:22 and 3 Nephi 10:17.
Summary: Restore in 3 Nephi 19:28 these in “thou hast purified these” since this is the reading of the original manuscript (which is extant here on a small fragment).