As explained under Mosiah 13:5, in the Book of Mormon text the possessive form for the name Moses is simply Moses’, not Moses’s. As expected, the printer’s manuscript omits the apostrophe for both instances of Moses’ here in 3 Nephi 27:8 (the first instance was miswritten as Meses). For the lack of apostrophes in the manuscripts, see the discussion regarding “three days’ journey” under the 1 Nephi preface.
Summary: Maintain in 3 Nephi 27:8 the form Moses’ as the possessive form for the name Moses.