Here the adverb away was accidentally omitted in the 1874 RLDS edition. The need for the word seems obvious given the phraseology “old things had passed away” just before in verse 2; thus the subsequent RLDS edition (in 1892) restored the away in verse 3. (In the printer’s manuscript, Oliver Cowdery accidentally overwrote the original y of away as a g, an obvious scribal slip.)
In this passage, Jesus is referring to his language earlier in 3 Nephi 12:47, where he said “old things are done away and all things have become new”. Note that the earlier text has a different verb (“are done away”, not “are passed away” or “have passed away”). But in both passages, the adverb away is there. Here in 3 Nephi 15 nearby references to this topic also use the perfect phrase “have passed away”:
We also get this same basic language later in the text when Ether prophesies about what will happen when the New Jerusalem is established:
Clearly, the adverb away is correct in 3 Nephi 15:3.
Summary: Maintain the phrase “to pass away” in 3 Nephi 15:3 and elsewhere in the text when referring to old things ceasing to exist.