In order to do a three needle bind off you need to have live stitches on the needle, as the front and back are bound off together. You can leave them bound off and sew the seams, or you can undo your bind off and do a three needle bind off.
The three needle isn't necessary, and many patterns don't call for it, but it does look nice and makes a very straight seam.
There is a video for a three needle bind off lower down on this page
It's really up to you what you want to do with the shoulder seams.