Matthew 5:8 (King James Bible) blessed are the pure in heart
When compared with the Matthew version, the Book of Mormon version has all added in five of the Beatitudes (as well as other words, also set in bold in the following):
book of mormon text
More generally, the Book of Mormon Beatitudes all begin with a narrative connector, either and (eight times, including one instance of and again) or yea (one time). The critical text will accept these differences, especially since they are so pervasive and obviously intended.
Here in 3 Nephi 12:8, the typesetter for the 1858 Wright edition accidentally omitted the all, perhaps because of his familiarity with the King James version of the Beatitudes. Yet only in this beatitude did he omit the all. The RLDS text, beginning with the 1874 edition, has in this case followed the reading of the 1840 edition, thus maintaining the all.
Summary: In accord with the reading of the earliest textual sources, maintain the occurrence of the extra all in five of the Book of Mormon Beatitudes, including here in 3 Nephi 12:8.