You've got your stitches divided on the three needles, right? Have them form a triangle so that the last stitch you cast on is next to the first stitch you cast on. Your working yarn will be on the right hand needle.
Use the forth needle to insert into the first stitch on the left needle (the first one you cast on originally) and use the yarn from the right hand needle to knit that stitch. Continue down the needle. When that needle becomes empty, use it to start knitting on the next needle and keep working your way around.