The 3 piece buttons are made up using two washers and a spacer that are assembled using a pan head screw when installing then on your rocket.
The easiest way to explain it is that It's basically a railbutton in kit form.
The one piece buttons are injection molded so no assembly is required. They also have a recessed screw, so they will be more aerodynamic.
Either one will do the job of guiding your rocket up the rail, so it's more of a choice convenience or style as to which you choose to use.