The 3-needle join is used to seamlessly knit together two pieces of fabric. It is similar to a three needle bind off, in that you knit together stitches off of two needles. However, the three needle join leaves you with live stitches, rather than a bound off edge.
How to work a 3-needle join
With one-half of the stitches in front and the second half in back, using a third needle, *knit the front and back stitches together, purl the front and back stitches together; repeat from *.