Diode with anode on two pins
I was wondering if it is in any way advantageous to use both pin 1 &amp; 3 (I am thinking more copper area for heat dissipation) or just one of them


To properly carry the power (Voltage * Current) for the diode, both pins should be used, with appropriately sized traces. But the anode pins are not the primary heat dissipation in a DPAK package. The Tab is. Look at the pad layout near the end of the datasheet. The entire back of the DPAK needs to be soldered on (reflow soldering of course). That's the primary heat sink away from the body, as well as it's body dissipation to air.


  and also what the purpose of a stub pin (not sure if this is the correct name) like pin 2 is?


The TO-252/DPAK comes in a variety of pin types, some with a stub pin in the middle, and some without (the pin is fully extended). In most cases it's always tied to the tab. While I can't be sure of what Vishey engineers thought in the design, it's likely due to two reasons. The manufacturing process for the stub version was cheaper likely due to scale, and as a way to force the use of the larger tab pin. Had the pin been fully extended, someone might decide that they will only solder the pins and not the tab, resulting in untimely heat related damage.