The 3-Needle Bind Off is a method of binding off that joins two sets of live stitches. It is often used at the shoulder to join the front and back of a sweater. In addition to being a bind-off technique, it is also categorized under seaming as it makes a neat, solid seam. If you hold the right sides of the knitting together before you bind off, the seam will be on the inside of the fabric. If you hold the wrong sides of the knitting together, the seam will be on the outside of the fabric.
How to do it
Hold the two pieces of knitting together with the points facing to the right. (The example shows wrong sides together.)
Insert the third needle into the first stitch on each of the needles knitwise, starting with the front needle.
Work a knit stitch, pulling the loop through both of the stitches you’ve inserted the third needle through.
After you’ve pulled the loop through, slip the first stitch off of each of the needles. This takes two stitches (one from the front needle and one from the back) and joins them to make one finished stitch on the third needle (held in your right hand).
Repeat this motion, inserting your needle into one stitch on the front and back needles, knitting them together and slipping them off of the needles. Each time you complete a second stitch, pass the first finished stitch over the second and off of the needle (as you would in a traditional bind-off).
As you progress along the stitches you will see the seam begin to appear.