A soybean cultivar designated SG3865N is disclosed. The invention relates to the seeds of soybean cultivar SG3865N, to the plants of soybean SG3865N, to plant parts of soybean cultivar SG3865N and to methods for producing a soybean plant produced by crossing soybean cultivar SG3865N with itself or with another soybean variety. The invention also relates to methods for producing a soybean plant containing in its genetic material one or more transgenes and to the transgenic soybean plants and plant parts produced by those methods. This invention also relates to soybean cultivars or breeding cultivars and plant parts derived from soybean variety SG3865N, to methods for producing other soybean cultivars, lines or plant parts derived from soybean cultivar SG3865N and to the soybean plants, varieties, and their parts derived from use of those methods. The invention further relates to hybrid soybean seeds, plants and plant parts produced by crossing the cultivar SG3865N with another soybean cultivar.