So you've cast on X number of stitches and you've begun by knitting along to the end of the first row.
Simply, as you say, cast on three more stitches (knit wise). Turn and Purl the first stitch, don't move it off the left needle and then just purl a second stitch.
Not as hard as it sounds - would help if you could scan your pattern (or an extract) so we can see the details?
Hope this helps?