Here in the original manuscript, Oliver Cowdery initially wrote “into the land”. Virtually immediately he corrected the the to this by overwriting the e with an i and inserting inline an s. In a number of cases in the text, Oliver accidentally replaced this land with the land; for a list, see under Alma 27:10–12. As also explained under that passage, there are cases of the land in the original text where modern readers expect this land. For each case of this land versus the land, the critical text will follow the earliest reading, here the corrected reading in 𝓞 (“into this land”).
Summary: Accept in Helaman 3:12 Oliver Cowdery’s virtually immediate correction in 𝓞 of the land to this land.