Matthew 5:34 (King James Bible) for it is God’s throne
The text of the Book of Mormon consistently avoids verbal contractions. Here in 3 Nephi 12:34, the 1837 compositor set it is as its, which should probably be interpreted as equivalent to it’s but without the apostrophe. The alternative interpretation, that its is the possessive pronoun, seems unlikely here since it is immediately followed by the possessive form God’s. In addition, if we accepted this second interpretation, there would be no be verb in the resulting text.
The 1840 edition restored the correct it is. But since the 1841 British edition was set from the 1837 edition, this error continued in the 1841 British edition and even further into the 1849 LDS edition. Finally, the 1852 LDS edition restored the correct it is to the LDS text. See under Alma 56:9 for discussion regarding the lack of verbal contractions in the Book of Mormon text.
Summary: Maintain the uncontracted it is in 3 Nephi 12:34; the original text (and current text) of the Book of Mormon avoids verbal contractions (like it’s).