Here the original verb form witness stands for the third person singular present-tense witnesses. In Early Modern English, the -es ending was sometimes not pronounced for words ending in -ess; apparently we have an example of this here in 3 Nephi 16:6. See under 2 Nephi 31:18 for a general discussion of this phenomenon in the original text of the Book of Mormon. The 1920 LDS edition made the change to witnesses, the expected verb form in modern English, but the critical text will restore the shortened form witness (the reading in 𝓟 and the 1830 edition).
Summary: Restore in 3 Nephi 16:6 the shortened third person singular verb form witness; such usage is occasionally found in the original Book of Mormon text.