3 needle bind off is only for top to bottom seaming (such as a shoulder seam) it is not used for side to side seaming. It needs live stitches to work, in other words your stitches need to still be on the needles. You can't bind off first and then do 3 needle bind off. If you were doing 3 needle bind off for a shoulder seam, you would need to have the same # of stitches on the front piece and on the back piece. With each set of stitches on its own needle, with the two needles side by side WITH RIGHT SIDES TOGETHER and needle points facing the same direction, you take you working yarn and a third needle and knit the first stitch from the 'front' needle and the first stitch from the 'back' needle together. You then slip the first stitches off each needle just as you would with regular knitting. So now you have ONE stitch on your needle in your right hand. Do the same thing with the second stitch on each needle in your left hand and now you have TWO stitches on your right needle. Now, you pass the first stitch (the one furthest to the right) over the second stitch and off the needle. Continue on in the same mannger until the end.
Now, another thing I would suggest if you haven't already done so is to watch Amy's video on 3 needle bind off. You will find it here and it may be more helpful than my wordy explanation!!
(it is the last one on the page)