The three-needle bind-off is a technique that binds off your stitches and sews a seam, all in one fell swoop. It’s a huge time-saver, and it makes for fewer ends to weave in when all is said and done (when compared to traditional seaming, anyway). It’s absolutely perfect for shoulder seams–firm yet elastic, and not too bulky. Of course, you’ll find other applications for it, too.
There are two rules to remember: (1) You need the same number of stitches on each needle, and (2) the right sides (or public sides, if you will) of the fabric always face each other.