The compositor for the 1858 Wright edition accidentally set have became instead of have become. Although the original text had some instances of became as the past participial form for become (see under 1 Nephi 17:43), this particular instance should probably not be considered the result of dialectal speech, but rather a simple typo. The earliest text reads have become, as will the critical text.
Summary: Maintain the standard have become in 3 Nephi 12:47, the reading of the earliest textual sources (𝓟 and the 1830 edition).