This is a handy little technique for attaching two edges of live stitches. Say, in a shoulder, or under the arm of a yoked sweater.
You could always just bind off and sew right? Sure, but binding off and sewing makes a lumpy, thick seam. Although not as invisible as grafting–aka Kitchener Stitch–[which will be coming soon!] the three needle bind off is harder-wearing while still giving a professional, clean finish.
Now, where were we? Yes, you’d just knit a swatch with a provisional cast on, which you then unzipped to reveal a bottom edge of live stitches.

Nice! Now, make sure you have an equal number of stitches on both needles. You will need to pick up one of those funny half-stitches at either the beginning or the end of the unzipped cast on row.
Since you want the seam to appear on the inside, fold the swatch so that the right sides are facing each other, and the needles are parallel to each other, as shown above.
Note: You can use any needles you want–straight, circular, double-pointed–as long as the tips of the needles are pointing the same direction.
Now . . . introduce the third needle to the other two needles by sliding it through the first stitch on each needle, knitwise:

See that? I’m knitting through two stitches at once. Therefore I will wrap the yarn and pull it through like so:

And then slide both stitches off their respective needles like so:

That wasn’t so hard, right? Now do it again. Just once more.

Now that you have two stitches on Needle Number Three, we can proceed to the binding off part. You probably already know what is coming next . . . insert the tip of either left hand needle into the second loop on the right hand needle and pull it over and off.


Now continue to knit through one stitch off of each needle and then bind the new stitch off.
When you’re finished, cut the yarn and pull it through the last stitch . . .

. . . and you’re done! Here’s your seam from the inside:

And from the outside:

Wasn’t that fun! Next time we’ll get to the Kitchener stitch (aka grafting).