Here Joseph Smith edited remaineth to remain since the subject is the first person I. Technically, the archaic -(e)th ending should be restricted to the third person singular, although in the original text it was very often used in the third person plural. Nonetheless, as explained under 2 Nephi 4:15, there were instances of this ending being used with the first person singular. The critical text will restore the original remaineth here in Mormon 8:3. For additional examples of this usage, see under infl al endings in volume 3.
Summary: Restore the original remaineth in Mormon 8:3 since use of the inflectional ending -e(th) with the first person singular pronoun I does occur in the original Book of Mormon text