The printer’s manuscript originally had the form beforetimes (but spelled as two words, before times); the 1830 edition has the form beforetime, without the adverbial s. In his editing for the 1837 edition, Joseph Smith removed this archaic expression, although beforetime (meaning ‘earlier’ or ‘in times past’) is found 11 times in the King James Bible (but only once in the original Book of Mormon text, namely, here in 3 Nephi 18:34). There doesn’t seem to be any particularly strong reason to remove beforetime from the text; perhaps there was some concern that beforetime might be misinterpreted as meaning that something occurred before time existed.
The textual issue here is whether the original text read as beforetime (the 1830 reading) or as beforetimes (the reading in 𝓟). The King James usage argues for beforetime (there are no instances of beforetimes in the King James Bible). But the Oxford English Dictionary gives examples of both forms in Early Modern English. Under beforetime in the OED, there are six citations of the word (sometimes spelled as before time); five of these date from about 1300 through 1614 and one from the 1800s:
Under beforetimes, the OED lists two citations, one of which is spelled as before times (as in 𝓟 for 3 Nephi 18:34); both these citations (original spellings here retained) are restricted to Early Modern English:
The OED lists beforetimes as obsolete but does not add this label to beforetime. This latter form was still familiar to speakers in the late 1800s (perhaps because of its occurrence in the King James Bible). Statistically, Literature Online shows that in the 1800s both beforetime and beforetimes continued to marginally exist in English, with beforetime about three times more frequent than beforetimes (statistically, 12 to 4). In today’s English, both forms are decidedly archaic. The most likely reading in 𝓞 was the more difficult reading—that is, the less frequent beforetimes—which Oliver Cowdery faithfully copied into 𝓟 but the 1830 compositor changed to the more familiar beforetime when he set the type.
Summary: Restore in 3 Nephi 18:34 the archaic beforetimes, the less expected original reading in 𝓟, rather than the more expected beforetime, the 1830 reading.