Here the printer’s manuscript has the do auxiliary (“he did baptize”) while the 1830 edition has the more standard verb form without the auxiliary do (“he baptized”). As explained under 3 Nephi 1:27, there is considerable evidence that Oliver Cowdery tended to accidentally add the auxiliary do. There are also examples of him omitting the auxiliary do. In fact, there is one example of this earlier in this same chapter:
On the other hand, the 1830 typesetter had only minor difficulty with the do auxiliary: there are no examples where he deleted the do, although there is one case where he added it (in 1 Nephi 17:1). Thus the critical text will accept the 1830 reading here in 3 Nephi 19:12 (“he baptized”).
Summary: Maintain in 3 Nephi 19:12 the 1830 reading without the do auxiliary (“he baptized) since the evidence is considerable that Oliver Cowdery tended to accidentally add the do auxiliary in his transmission of the text; on the other hand, there are no cases where the 1830 typesetter omitted the do.