As discussed under Mosiah 27:13, the text is evenly divided between the singular and plural for transgression when immediately preceded by their. We therefore follow the earliest textual sources in determining the number for their transgression(s). Here in Alma 3:6, scribe 2 of 𝓟 initially wrote the plural, then immediately erased the final s. As noted under Mosiah 27:13, this scribe tended to incorrectly write the plural transgressions in place of the singular. The critical text will maintain the singular transgression here in Alma 3:6.
Summary: Retain in Alma 3:6 the singular transgression, the corrected reading in 𝓟.