Why does this schottky diode have three pins
In general, single diodes in 3-lead SM packages have no connection on the third pin. This is a fairly common practice in the industry.

I don't have a solid answer as to 'why' they do this, but will share a few possibilities for debate:


You cannot install a single diode in a 3-lead package with reverse polarity because of the asymmetry of the package; if the diode were in an 'inline' 2-lead SM package (like SOD323) there is a chance of reverse installation
3-lead packages are larger, allowing for larger die sizes (better thermal performance), easier installation by pick-and-place machinery (generally, the smaller the part, the trickier installation is) and easier optical inspection