As discussed under Jacob 3:10, the original text of the Book of Mormon had a number of cases where the verb sit was used transitively, as here originally in 3 Nephi 17:12: “they … sat them down”. In this particular case, the 1841 British edition, the 1874 RLDS edition, and the 1920 LDS edition have each independently made the change to the standard transitive verb set. Earlier the 1849 LDS edition restored the original sat to the LDS text, and the 1908 RLDS edition restored the sat to the RLDS text. Since 1920, the LDS text has maintained the standard set. The critical text will restore the nonstandard sat since it is the reading of the earliest text. For further discussion of this usage, see under 1 Nephi 11:1 and, more generally, under sit in volume 3.
Summary: Restore in 3 Nephi 17:12 the past-tense form sat since it is the reading of the earliest text; transitive uses of the verb sit occurred several times in the original Book of Mormon text.