The three needle bind-off is a useful technique to acquire a nice, neat seam while avoiding having to sew it up.  It is quite strong and especially useful in the shoulder area of a sweater which get a fair bit if stress.  It will feel a little awkward at first but with practice, it becomes easy to do.  In addition to the needles you’re working from, you’ll need one more needle in the same size.

Ordinarily, you would be joining two separate pieces of knitting together - the front to the back of a sweater, for example.  Here, with a 20 stitch wide piece of stockinette, I’ve knitted 10 stitches and folded the piece.  Make sure the right side of the pieces are on the inside - you want to be looking at the wrong side of the knitting.  Make sure that the needles are parallel to each other and that they both point in the same direction.  The working yarn should be coming off the first stitch of the needle closest to you.

Insert the tip of your third needle into the first stitches on both of your stitch-holding needles and knit as though you were doing a k2tog, removing both stitches from their respective needles and creating a single stitch on your third needle.

Repeat the procedure so that you have two stitches on your third needle.  Lift the second stitch over the first as you would with a regular bind-off.  Continue in this fashion to the end of the row and fasten off.

Presto!  You have just made a nice, neat, sturdy seam.  Not too shabby, hunh? 

Some folks pick up the stitches along the sides of a garment and bind them off this way to work the seam.  I haven’t tried this myself but it’s an interesting idea and would probably be especially useful on children’s garments.