Matthew 5:35 (King James Bible) for it is his footstool
Here the 1830 edition lacks the linking verb form is. The original Book of Mormon text sometimes lacked the linking verb be, but only in the Isaiah quotations (see the examples listed under 2 Nephi 13:14). Elsewhere in the text, there has been the occasional loss of this linking verb, as here in 3 Nephi 12:35 where the 1830 compositor, it would appear, omitted it. His eye may have skipped from is to the visually similar his that immediately follows. 𝓞 probably had the is, although there is the possibility that it was lost when the text was dictated. In any event, the printer’s manuscript has the expected is, as does the corresponding Matthew passage. The 1837 edition restored the correct reading to the printed text. For other examples where a typo has led to a Hebrew-like subject complement structure without the be verb, see under Alma 41:13.
Summary: Maintain in 3 Nephi 12:35 the linking verb form is, the reading of 𝓟 (“for it is his footstool”); this reading agrees with the Matthew reading; the 1830 edition lacks the is, but this appears to be a typesetting error made by the 1830 compositor.