It sounds like you bind off 3 stitches at the neck's edge and use whatever decrease method you were using before for the armhole. I would need to read the pattern to know for sure, but that would be my best guess. Also, patterns tend to come with images of what the pieces look like before you sew them together. I would use those images as a guide. Having knit 2 adult sweaters and 4 baby sweaters before, not a single one had the same kind of neck shaping.