Scribe 2 of 𝓟 initially wrote the singular Border; shortly thereafter he inserted a plural s with heavier ink flow. The original manuscript probably read as a plural since scribe 2 is not known for his editing of the text. Moreover, in the Book of Mormon text we find examples of only the plural borders but none of the singular border (for further discussion and examples, see under 1 Nephi 2:5). The critical text will maintain the plural borders here in Alma 5:3.
Summary: Retain in Alma 5:3 the plural borders, the corrected reading in 𝓟; the text otherwise uses borders, not border.